Get the price of a well looked after supra with no costs attached to it for fixes and maintenance, then subtract the price of the supra you are looking to buy and multiply that by a number equivalent to the supra inflation rules.
I bought my supra for £3k less and spent about £6-7k on maintenance in the first year. Thats excluding hassles and things I didnt nessecarily need.
Worked out in the end as the supra is now worth more than ive put into it.
Annual costs tend to be £1-2k for maintenance although I am prepared for it to creep higher at any time.
It obviously helps to do a lot yourself.