Your hardest thing to find is a good source of beef, but you can get locally source beef from local butcher (most of them are islamic) for between 250-320bt/kg. Otherwise you'll have to pay almost 1000bt/kg for imported stuff which you dont want.
Pork and chicken obviously not a problem, if you don't like buying stuff at markets cheap (and you should, its fine), then get the 'hygienic' stuff at tops for about double the price, which is still OK.
Fish - again you cant go wrong with salmon - around 600bt/kg is the cheapest you will get. Do consider eating some of the local farmed perch or 'seabass' - about 70-100bt/kg for the whole fish it bakes very nicely and the flesh is white and soft and nice.
There is also, if you search for it, a couple of good quality sausage and smallgoods vendors in BKK - if you want to eat real meat sausages and ham and bacon that doesnt cost the earth.