Estimates are just a cost of doing business in my opinion, they should always be free. Especially because during that time you are most likely selling yourself/services and not just giving an estimate. The unfortunate bit about free estimates, is if you aren’t willing to do it, there are several other competitors who will. I would count it as a learning experience and continue to shine through your work. You don’t want to miss out on opportunities because a competitors is always waiting to swoop in and take your potential customers.