What is the best affordable interior latex primer?

Kilz is as good as any other brand of primer. If you want something thick, don't purchase primer, instead purchase paint made to cover water and oil stains.

All primer is just paint thinned to half consistency of the normal paint. Most professional house painters will make their own primer by diluting paint. Putting primer on thick just waste time and money.

Primer is necessary ONLY if you have new dry wall or wood or other material, or your project has peeling paint. The purpose of primer is to seal the surface of the material you intend to paint so it won't keep soaking up paint like a sponge. Once the primer is dried, it forms a surface that the finish paint can adhere to and not be absorbed by the object being painted.

If the object has been painted once already, the material is sealed and another coat of primer is a waste of money and time. It does absolutely nothing since the object is already sealed and can't absorb more finish paint.

Ranger is right -- listen to him and give him the points. I worked for a while for a couple of professional painters and have done a lot of it myself. You want a thin shellac based primer like Kilz to seal the surface and block stains and color bleed. Then top it with a final coat of quality paint -- I've been happy with Behr paint from Home Depot. If for some reason you need a thicker top coat (like for textured walls or dark color overlay) go to Sherwin Williams for paint.