There are always lots of variables to take into consideration when being a subby. Some of these would be the location that you live in comparison to the area that you are prepared to travel to get the work.
Whether you are a known engineer with an established background of quality work.
How keen you are to get some good work to build your reputation.
Sometimes cheap is not always the best, but you have to start somewhere.
If you are working on your own you might be able to ask for up to 200.00 per day without materials, but that would include mileage up to say 60 miles. if you can provide a reliable mate, then if you have an established background you might be able to get 300.00 per day including mileage.
Pricing jobs including materials will take you a while to get to grips with the materials costs. If you do not have any trade accounts, then you will be paying more for your materials. If your business is not REFCOM registered then you will probably have a problem buying refrigerant legally.
You will learn quite quickly the companies that pay you and give you more work and the ones that give you more work, beat you down on the price and then pay very slowly, if at all.
It is a steep learning curve which provided you can give good quality workmanship and value for money and don't give in so that you can build your reputation.
Good luck and just take it one step at a time.