I remember a thread on this last year prior to Paris. I think all you need is a letter from a GP saying that there is no reason why you shouldn't enter the marathon, rather than a full medical. Suggest that to your GP and see if the cost falls. Alternatively, some people wrote their own and sent them off, and were not questioned. Depends on the race and whether they check them out. I think they ask for them to cover themselves in case of a claim, so if you have "forged" one, it's your lookout, much the same as the disclaimer we all sign when entering races in the UK.
