A FAQ, "What should I pay?" Can simply be answered by telling you to pay only what you think the item is worth. If you don't know what it is worth, you can easily find out by asking the price at several locations.
The first price is usually inflated and when you turn to leave the second price will be quoted. Sometimes the seller will ask what you want to pay. If your answer is so low as to be insulting, you will not get that item from that seller for less than the originally stated price. After stopping at several shops you will get a feel for the bottom-line price and then can easily make your choice.
There are several fixed price stores on Revolution and at these, it will do no good to try to get a lower price.
TJ is a great place to do some shopping. You will actually get a better deal if you use U.S. dollars so don't bother changing your money into pesos.
The San Ysidro border crossing is among the busiest in the world and Tijuana is the most visited border town in the world. You can expect it to be relatively packed with tourists and the sights and sounds are definitely different that what you'll experience in the States. Just keep in mind, not all of Mexico is this tourist-oriented.
Favorite thing: There will be no end to the shopping opportunities in TJ. Just walk around and never buy anything without doing a little bargaining.