This was the cheapest one I found. Personally, I use this one, but I bought it used for ~$70. If you're already at 15 people, you may want to invest in a 24 (or more) port switch, to give yourself some room to expand.

As /u/sirsquidness said, the 10/100 switch will work fine for running games, it'll just be slow if everyone's getting their steam updates from the cache or such. If it's a one-off, I wouldn't bother, but if you're planning on running another, it'd be worth the extra initial cost.