Best bang for the buck. Hmm. I would start with caliber, 556/223 or 7.62x39. The first will run you around 80 cents/round, the latter 40 cents for corrosive. It really depends if you intend to shoot a lot. 556/223 is more accurate if that's your thing.
Best bang for the buck? Used or new, if new and you want the lowest price, subscribe to theammosource email notification and wait for their sales (15 to 18%) or check out arsenalforce for a search. Prices can be much higher if you want to buy NOW.
Cheapest is a slightly used WK180 for under $1k in 556/223 or a Type 97 gen 1.
The type 97 is a bullpup as well as the Tavor X95 for 3 times the price. The Type 97 is not available new anymore. Both are in 556/223.
As to 7.62x39, there is the WK181 which is reasonable priced and there is the CZ Bren 2 for 3 times the price.
Depends on you what you consider best bang for the buck.