You will need 2 more Network Interface Cards (a total of three NICs), a Network Hub, and some CAT5 ethernet cable (do not get crossover cable). You'll a total of three cables (one from the DSL modem to the first computer, one from the first computer to the hub, and one from the second computer to the hub).
There are two basic types of NICs. There are 10baseT cards that can transfer data at 10Mbits/s (1.25 MBytes/s). And there are 10/100 cards. These cards can transfer data at 10Mbits/s or 100Mbits/s (12.5 Mbytes/s). Your DSL modem will never even reach 10Mbits/s (a T1 line is 1.5 Mbits/s, or 192 KBytes/s). So a 10baseT will work fine, but I suggest getting a 10/100 card because they're about the same price. Plus, when you transfer data between your computers you will be able to use the 100Mbits/s speed. NICs cost about $20 each.
The same basic rules apply to a network hub, except 10baseT hubs are less expenive. You can get one for about $25, compared to $50-100 for a hub that runs at 100baseTX. Mine is a 10/100 auto-sensing hub. That means that it will run at 10baseT or 100baseTX depending on the NIC that it is connected to. It is a Netgear that I bought over a year ago for about $75 at Circuit City. It's probably less now.
A lot of companies make networking kits that come with everything for $75-150, well below your limit. If you wanted to buy it online, you could get it even cheaper. Search on <A HREF="http://www.pricewatch.com" target="_new">http://www.pricewatch.com</A>.