It used to cost about 50p/mile for my 1.8 16v when it was nearly new but you're talking about an older car. The depreciation will be less than new, so comparing it to a 147 might not be that accurate.
You should be able to come oup with some rough figures of your own...
estimated depreciation (esp. considering extra mileage)
insurance (will you need to pay more for business?)
service costs (extra due to mileage)
general wear and tear (will business use increase this through carrying loads?)
Also, if you receive a mileage allowance you may be taxed on the "profit" element. For example, I get about 50p a mile for work and am taxed on that!