she's very pretty
price is HARD
two tips
#1 reason for making customs should be because you want to and for fun and not because you want/need to make money off it. It won't work in your favor to do that.
Also, you can always lower your price, so start with a $ amount you feel good about, and decide how long you'll wait to see if it sells. Some amazing artists sell their art for way less than they could get because they want it to sell within a week or two. Others are ok keeping it indefinitely, and may or may not lower price or have sales.
I've seen some people suggest calculating the cost of your supplies and adding some round number to it ($20, $30, etc). If you're already thinking about how many hours you put in, see my #1 point above. You won't make anything for your time doing this. Some of us spend months on a custom hahahah
My best advice on pricing for someone getting started at establishing their price point is to look at what customs are actually selling for on ebay/etsy/mlparena, etc and try something similar, knowing you may need to mark it down at some point. If your pieces start selling well, then you can raise your prices. I also suggest that sculpted work on ponies increases the price over "just" painted
It is a personal decision because it's your art. And art and business don't see eye-to-eye in our culture. In the end, it's worth what someone will pay for it, and connecting your art with the right person can be challenging
And one last point -- use social media to promote and try different selling platforms