The cost of the crank seal is no big deal, it's part of the center main bearing and the bearing is £37, the O ring seal is very cheap.
The issue is (obviously) fitting it as it's in the middle of the crank, I'm not going to say that it can't be done without fully stripping the crank but it wouldn't be easy, may not be wise and there's a good chance the rest of the crank isn't 100% either.

The reason it affects starting more than running is (presumably) as if the seal isn't sealing then primary compression isn't ideal, the slower the motor is turning the more the problem shows (as more mixture has time to leak through the seal) and when it's being kicked over it's turning very slowly indeed.
_________________
Please do not PM me technical questions, if you can't find it on the Forum start a thread