They are not quite interchangeable, although I just checked a dictionary and it defines "lucrative" simply as "producing wealth; profitable." Nevertheless, I think there's a difference. "Profitable" is a plain, factual, ordinary word and it's the one you should usually use. It simply means that the contract is expected to make a profit for the company. You can intensify it by saying "very profitable." "Lucrative" carries a sense of great, _possibly excessive_ profit. I imagine it being used in a context like this: "Well, we got the contract! And I can't believe it, they just accepted our numbers for the second and third year without trying to negotiate them down!" "No kidding! Wow! Well, congratulations! I think we should go out and celebrate, that's going to be a lucrative contract for us!" By comparison: "Well, we got the contract. But we had to make some concessions, they're insisting on a 10% reduction for year 2 and a 20% reduction for year 3." "Hmmm. Well, that wasn't what I was hoping for, but it still should be profitable for us."