Never base it on Equipment you have. I have plenty of equipment, but the market where I am, is slim. There's a few artists here that use my studio, and I charge about $30 an hour for recording, and about $50 an hour for mixing/mastering.
You can claim you have skills mixing and mastering, but do you have proof? Do you have a list of clients who've used your services, and will vouch for your product?
I did quite a few free projects for artists, so I could build clientelle, and product. It's a living resume that I'm able to show prospective clients as proof of my abilities.
I also work out deals, based on the amount of recording/mixing/mastering they need. If they want to do an entire project in my studio, I'll cut them a crazy deal, because I know I'm in control of the entire process.
If I'm mastering someone's project which is already mixed, I'll charge them by the hour. No discount... unless I know them.
Every situation is different, but the main thing to remember is, do you have proof of your abilities and the quality of product? If so, you can charge whatever the going rate is in your area... If not, you need to build up clientelle and product before you can start making any coin.