Cheapest is just a rack suit at Kohl's or some department store like that. You'll need to get it altered after though so save money for that. Hit or miss on the help at those locations so make sure you can fit yourself decently.

Got my first suit at Rocco's and they were amazing. Asked about my budget and anything I knew I wanted, gave suggestions without pushing me into spending more. Do alterations as part of the purchase. Might spend a little more but without experience I think it was all worth it long term.