Since quality comes with a pricetag I find it fair to work on time wage and all my prices are based on a calculation of how much time it will take me to finish the project. I will work for two BigMac per hour (whatever they cost at that time) and as long as it needs to get a model done. Painting a single mini takes me about four hours, converting varies greatly but is almos never under five hours.
To give you an idea on how much I earn with each model you can take a look at the Big Mac index (the BigMac currently costs € 3,30- at my local McDonald's.)
If you want a comission just leave me a message here with mail adress and I will gladly get in contact with you.