most probably this might be a keyboard problem. regarding the prices and the exact cause of the problem its best if you contact dell customer support. [i had the same issue and got it resolved pretty quickly]
Well it really depends on the problem, if the key is broken or missing you should be able to just replace either the retainer clip or replace the entire dell laptop key, if its liquid damage, then you need to get a new keyboard. For best answer, we need more details.