When do you need this by? I think if you're patient, you can do pretty well. I have a Dries suit that was $400 and a Jil suit I got from a seller here for like $70 (the Dries suit is perfect, the Jil suit I still need to have tailored). Of course, I've never had a need to wear either, but I'll assume I'll be glad I own them at some point?
Yoox and ebay are both great for this, although ebay requires you to be willing to re-sell a bunch of stuff that doesn't end up fitting.
There are a few people on this forum that have a pretty big presence on ebay that come to mind like spoo, brianpore, jgill etc. These guys know their measurements and are reputable. Point being, if you can figure out what fits you and have reasonable measurements, you'll easily be able to get something that will cost less than 300 and fit well with some tailoring.
Yoox is a great suggestion as well, but the sizing for stuff can be all over the place and I guess you have to stick with what you know or be willing to order a few sizes and play the returns game.
Also, formal AE shoes pop up on ebay ALL the time. It's a mass brand and you can get park avenues for <100 for burgundy which I prefer to black.
You'll be able to land shoes and a suit from ebay for under 300 together if you give yourself a month of looking for stuff.