If you want to start with something really cheap so you're not making too big an investment while you're first dipping your toe in the waters, the Walter Klien (and it is "Klien", not "Klein") Piano Sonatas on VoxBox are superb, really! There are two sets, each of which consists of 2 cds for all of $10.98 currently on Amazon. Hell, that's less than Naxos these days (and I don't mean to dis Naxos). I'm linking Volume 2.

In general, the problem with Mozart recordings (and those of other famous composers) is that there are so many to choose from that it's hard to decide. The flip side of the coin is that, because there are so many choices and so much competition, there are a number of outstanding older recordings (generally late 50s through mid 70s) that are mid-priced or less. There is rarely any need to pay anything close to full price for a fantastic performance of any "name" composer, unless you insist on a "DDD" recording.

• Klien, Mozart Piano Sonatas Vol. 2