Valuation of Parking Spaces

[Chris Carson]

Hello everyone, I have a unique opportunity and have never dealt with anything along these lines so here goes:

My law firm owns a very nice space in a fairly large office building. The first 5 floors are parking and the top 6 floors are commercial office space. There are some vacancies (est. 10%) in the building but it's in a fairly good area for businesses. An owner down the hall  recently underwent foreclosure and lost their office space but still owns 15 parking spaces free and clear. They have offered them to us as a bundle. We could use a few more for our office but don't need that many. Currently spaces rent for around $100/mo. and taxes run about $100/yr. How should I be looking at this in terms of valuation? Anyone ever done anything like this?