And a Scot - we're known for being tightwads ...
It all depends on your size. If you are unfortunate enough to fall into the size range 38R-42L you will be pretty much restricted to above $100 for a suit on eBay. If you're bigger than that, expect to pay more. Smaller? You might get lucky. The real trick to eBay is finding things that have been badly listed (look for measurements that seem absurd to you, or for a suit that you *know* is 30s-40s but has been listed as 60s-70s). The threads on the lounge will help you out with what to look for.
Other than eBay, the internet is hit and miss. Trawling through vintage boutiques you'll find anything from the absurdly high prices (one has a 40s two piece suit for $3500 - and it has moth holes) to the absurdly low prices (NRA overcoat for $15) and everything in between. For a three piece suit $100 seems to be around the going rate.
I thrift obsessively. there are two Goddwill stores in my town, both of which i visit once a week. I go to every store in town that sells vintage at least once a fortnight. My finds from the town currently run to a wrecked 30s tuxedo jacket to use for a pattern, a 40s herringbond overcoat, a 1940 dated two piece suit from the goodwill, and two stetson hats. And i've been searching for 3 years. eBay has provided most of my other vintage clothing.
Building a vintage wardrobe on a budget takes a loooong time. With unlimited cash one could quite quickly amass a mass of clothing, but most are not in that situation.
Keep on thrifting!