How much for mix on demand?
 

A guy asked me to do an easy mix between 10 tracks, 2 minutes each.

He got midi files, so i need to use vst unstruments to bring them to life, then do the mix.

How much i have to ask for the whole operation?

I always think that successful technical consulting (doesn't really matter its music, software engineering etc) is really about 99% level-setting expectations. So for both of you to come out ahead, you need to get on the same page as far as what you're signing up for. The outcome of converting MIDI files into a listenable track could be a 5 minute job that sounds like crap or dozens of hours per track achieving some desired result.

What I would do in this case if I were you is decide how much your time is worth to you, per hour. Arbitrarily lets say its $50 per hour, but do not give an estimate on the total amount of time it will take until you've been able to assess all the MIDI files. They could be a real mess. Once you have all the MIDI files, find the one that looks hardest to work with. Then from that, produce two different .mp3s with a duration of about 20 seconds each, one which takes you five minutes to do but probably sounds like crap. The other one represents your absolute best level of work with meticulous tweaking and using your best mixing know-how. Provide these to the client as samples, showing the variance in quality, then let them use that to gauge the level of quality they prefer.

You only have one unknown at this point which is the time-to-develop variable. You can then base your time estimate by how much time you spent on each and how good your client wants them to sound. Done deal.

Thanks MBTC, your interpretation sounds good to me.
In addition with the word mix i mean a dj-like transition between songs, which can be made with midi material, but takes more time...

I will do your time evaluation based on quality.

Right on, and good luck. If you can, get non-exclusive rights written into the deal and share with us the end result ;)