Buying used truck owners says it needs lifter valves what am i looking at in cost

If it needs lifters and valves, I would suspect the rest of the engine is in the same shape. My guess is you would be looking at a couple thousand dollars to rebuild the engine.

In order to replace the valve lifters, the rocker arm covers need to be removed, Rocker arms and push rods as well as intake manifold. if you have an overhead cam engine it's a bit different but should be about the same amount of labor hours. Price varies with each shop and parts cost depends a lot on exactly what is found after the engine is apart.
Unless you have a competent technician look at and verify that the lifters are the only problem, you may be getting into something very costly. Lifters can fail in two ways: The internal plungers can get stuck or internal clearance may cause excessive pressure bleed off causing them to get noisy. Or, the camshaft (the lifters ride on that) may become worn and cause excessive clearance that the plunger inside the lifter cannot compensate for. That requires a cam replacement which is expensive.
One other item: Good lifters can become noisy if the engine oil pressure is low. The primary cause of low oil pressure is worn engine bearings. In that case, you'd need a complete engine rebuild or replacement.
An experienced tech can do a pressure test using a mechanical pressure gauge to verify the actual oil pressure and he can advise you on what he thinks about that and the overall condition of the car.
Personally, I would not buy it unless it's in exceptionally great condition and I had the time and $ to invest in doing the repairs. There are lots of good vehicles out there and buying one with obvious problems is often the beginning of something you don't want to do (like cleaning out your bank account).

You have to remove the cylinder heads to replace the lifters, depending on were you live $800- $1500