I'm in law school and depending on where you work you can make a lot of money if you work at a fancy national/international firm, but if you don't you don't actually make that much regardless of your quality/skill. Gov. jobs and private interest currently usually start at around $75,000 and a lot of lawyers have at least $100,000 in student loans. Even if he got a good scholarship, he would likely have debt from cost of living loans. Plus, Cam doesn't work (until later in the show) and adoptions are very expensive.

Realically, though he was top of his class at a top law school and could fairly easily get an amazing job if he wanted. Plus, I think Columbia law school has a pretty good loan repayment program for public interest lawyers (although it could have been different then and that's not where I go, so I'm not 100% sure).