Hey Coffee Head:
Some people in this forum may not be shop owners, etc. and others may not know how to answer this question. First off, where are you located, and why would you build a building first and plan to place it next? It should be the other way around. A 15X20 is a good size building and will require a lot of land. Next I would ask, why rent the land instead of purchasing it? I would say look for land to purchase then you can have all of the say so, and not risk the chance of getting booted off if the property owner decides to sell. My sugesstion would be to find a commercial realitor to find a parcel for you. You'll be happier if you did. Give that realitor a list of things you are looking for including traffic count, demographics, etc. You can always start off with a good lease with an option to buy. That's where I'd start. But your question about what a good rent for land is not an easy one to answer.