Lifters & Valve Noise

I bought a 99 Malibu that has "lifter"? noise.

A mechanic told me it appears to be the exhaust lifters; I buy the lifters, he replaces them, says that's not it.

He wants to do a valve job on it...says it already had intake lifters replaced.

The costs just keeps climbing and climbing

Is this valve job more than likely going to be the end of it? How much is a valve & exhaust lifter job worth nowdays?

Thanks for any help you can provide...