I'm sure you've looked at all the figures already, but I guess it would break down as follows;

Rent of unit per year = x
Rates per year = x
Cost of insurance = x
Cost of manager per year = x
Maintenance costs of tools and equipment per year = x
Cost of running recovery truck per year (tax, insurance, repairs, MOT etc.)pre year = x
Cost of consumables (welding gas, wire, saw blades, sanding discs, so on and so on) per year = x

Total figure for the above = y + 10% for contingency. Divide that figure by 12 and then by 4. Which gives you a break even figure per month, but doesn't include your salary.

Then you need to decide how many days a week you want to open, or are legally allowed to open and then the number of hours per day, per day of the week/weekend you want to open for and are legally allowed to open for.

You also need to take into consideration that if you are open from 8am until 10pm Monday to Friday (later if jobs over run) 8am until 6pm on a Saturday and 10am until 5pm on a Sunday (being that weekends and evenings will be your target audience for this sort of enterprise), that you will probably need to employ a third member of staff, adding additional cost.

Once you have worked out all of the above, it should give you a rough cost per hour on a break even basis, provided you could fill the workshop from morning through to night.

This should give you a base to work from, you should also have an idea on number of cars you sell per month year and the amount of service/repair work you get per week/month.

I guess in times like this you need to consider other things as mentioned by other posters, such as car washing and the like, you could also look at becoming a re-seller for tuning products such as alloys, tyres, stereo equipment, exhausts, suspension kits and so on to ramp up your customer base.

The more you can offer the more customers you will attract, as well priced one stop shops are always good business, regardless of the type of business you are in.

Good luck