Determining Target City(ies) for Marketing?

Hello!

I'm the co-founder/engineer for the site https://fixmybikeathome.com. I'm wondering how to determine how many cities to advertise to on a specific budget.

Our site connects local bike mechanics with customers that want their bike fixed at home. With an unlimited budget, we would obviously just advertise in every city.

With a limited budget I'm presented with a variety of options... Spend a lot of money advertising in a single city, or spread the amount of money over 1-5 cities. It's important to have both mechanics AND customers on the site in the same city, so my inclination is to do as we have been doing and advertise in 1-3 cities.

If that is the correct course, how do you determine when to expand? If you have any book/article recommendations on this topic, please send them my way!

Thanks,