You could also do a "Chinese Auction" where people put as many tickets as they want to bet on a horse. Use a separate box for each horse.
http://en.wikipedia.org/wiki/Chinese_auction
For example, a guy with 10 tickets can split them up any which way, putting 4 for horse A, 3 for horse B, 2 for horse C, and 1 for horse D.
In this application, the Chinese Auction will become a mini - parimutuel system.
To do 50/50 here, just give half the number of tickets, so $20 buys 10 tickets. At the end, just divide up the non-winning boxes of tickets by the winning tickets. You've already done the "takeout" at the buy-in. Round down for "breakage".
Not as simple as the 50/50 but still has potential.