As a handloader since the 1970's,, I can say that the OP has asked an honest question, and been given a lot of excellent info.
But I want to address his initial comments about the cost savings,, and to quote him; "to spend possibly thousands on reloading equipment and supplies."
And he later says he's considering a Dillon SDB as a starter set-up. A SDB new price from Dillon is $630.00. it comes with (1) caliber set-up. (In the OP's case,, .38 Spl.) add to that the powder & primers,, as mentioned,, around $350.00 for powder & primers. Buying bullets,, will increase that expense, (from about .10 cents per bullet to upwards of .30 cents a bullet, depending upon what type & where he buys them,) and if he already has brass he'll be set up to just do .38 Spl.
By using those numbers,, yes,, the initial expense will exceed $1000. If you only loaded 50 rounds,, that'd be a very high expense for just 50 rounds. BUT,,,, if you loaded 1000 rounds,, (using the number of the primers mentioned, then the cost will be close to what 1000 rounds of factory ammo would have cost. Here's the point the savings kicks in. At 1000 rounds,, you've about broken even on the expense of setting up. But now,, you are out of primers, and lets say for the sake of discussion, just purchased 1000 bullets initially as well. Now buy another 1000 primers & bullets. What did that cost? Maybe $350-$400?? The powders most often used in .38 Spl if purchased in an 8 lb jug will load around 4000-7000 rounds,, so you will still have powder. Your second 1000 rounds will be much less than the first 1000 rounds.
Over time,, depending upon how much you shoot,, you will see the expense of the ammo settle in at around 1/2 the cost of factory ammo as an average figure. You can reduce that buy the way you buy components,, by watching for sales,, getting a good deal in various locations, etc. And if you were to find that you really enjoyed the hobby,, you could try casting bullets to make the bullet expense even lower.
And after all that,, you still have a great reloading machine that has value. A Dillon progressive retains good value,, while others,, maybe not as much. I learned long ago when it came to equipment that I was going to use a lot; "Buy once, buy quality, and you only cry once." Good quality can outlast our lives. I have a RCBS RockChucker press I bought used in 1980,, that is still a top quality press,, and I can easily sell it for a lot more than what I paid back then. The same goes for my Dillon presses. I have less in my 1st Dillon 650 with casefeed than the current cost of the new SDB. I know I could make money on the set-up.
Now the negatives.
The Dillon SDB is a great machine,, but uses Dillon only dies. As mentioned above,, the 550 or 750 etc can use standard dies. If you ever have any idea of reloading any other calibers,, this is a bigger deal, especially if you might load any rifle calibers. the SDB doesn't load any rifle ammo.
Another negative,, especially for a new reloader,, is the learning curve of a progressive (the Dillon) over a single stage. BUT,,, they you can begin on a Dillon,, and do quite well. if you read the manual, and especially if you have a friend who can mentor you in their use. A single stage press is much slower,, but it does allow a new loader to slowly learn the process, AND understand what is happening. Another good fact of a single stage,, is when you have a problem,, a single stage is an easier & quicker "fix." Just the other day,, as I was using my Dillon 650 and loading some 9mm ammo, I had a few little problems. I was loading 600 rounds of 9mm, and during that session, I had a couple of case that the primer didn't get knocked out completely. For me & my experience level,, it wasn't a big deal. But a new reloader may experience a lot more frustration over that.
Since the expense of loading seems to be the theme of the OP,, I would suggest a bit of careful & judicious shopping around. I have helped a few friends find & buy used Dillons for very good prices. In just the last year or so,, (4) SDB's all cost $300 each. And a couple of them came with extra caliber conversions. Then a 650 & a 750,, each sold for $750 with case feeds etc. (A member here got the 750.) The point is,, you can get top quality,, without paying retail. And yes,, Dillon still has their lifetime warranty,, even if you didn't buy it new.
But a Dillon is the best progressive machine available to the hobby market. And yes,, is does allow for much faster loading of any amount of ammo over a single stage press set-up. And if only loading (1) caliber,, in handgun calibers,, the SDB is a good one to use. Initial expense seems high,, but as mentioned,, you will recoup it the more you use it & keep it.
In looking at the current Dillon catalog,, a SDB costs $629.00. But if you add the stuff in the picture to "upgrade" it,, (the strong mount, the low powder sensor, the bullet tray kit, the tool holder & wrenches, and the case bin & bracket,) it lists for $948.00.
For a beginner,, those upgrade are NOT "necessary." If you want the bullet tray,, then the strong mount & the case bin & bracket are required. But if you mount the press to a bench,, (mine is) not necessary. In fact,, all the accessories except the low powder sensor require the use of the strong mount. And While I have (4) Dillons on my bench, as well as several caliber conversions,, NONE,, and I mean NONE have the low powder sensor.
At $629.00 plus shipping,, you can buy powder, primers & bullets,, & start loading once it's set-up & adjusted. Beyond that,, you will NEED at least (2) necessary items. A powder scale, and at least (1) good reloading manual, (better to get a couple of different manuals though.) I like to say that after 10,000 rounds of loaded ammo,, in general,, you have reached the point where the expense of the equipment has been met. Then the only expenses are the expendable components.
Having the equipment and enough components can allow you to shoot when others can't because ammo dries up, or components dry up.
I'll close this long-winded response with a statement made a few times above. You may not save money,, but you can shoot a lot more for the same money. But if you shoot the same amount as you currently do,, then yes,, you will shoot for less money.
