There are a few HOFers (Doerr and Feller to name a couple) who signed a ton of autographs, and as a result are available for cheap. Just take some time with ebay to find out which guys are cheapest. If I didn't already have this picture of Feller (my favorite of him by far) signed, I would want one, and they are readily available at a low price:
http://www.ebay.com/itm/Bob-Feller-s...item417fd718fc
disclaimer: not my item, and there are plenty of other sellers that have one for sale
Also, if I were to start an autograph collection from scratch, I wouldn't focus on just balls, or just 8x10s. When you get into collecting pre-war players, the autographs tend to be on smaller, oddball items like postcards, 3x5 index cards, programs, checks, etc. If you're ok with collecting smaller oddball items, you can find some good deals and build a fun, well-rounded collection.