It depends on where you are booking accommodation. I stay in Washington, and on the morning I drive to Hewarth metro and park there (think it's £1.20 for the day), and get the Metro into Newcastle. It's much easier and less hassle. I wouldn't park in South Shields unless you want to spend a lot of time in the area afterwards.
I don't know about parking in Newcastle itself, but one of the regulars might know.
Exactly RLTW - London was £35 unaffiliated last year I think, but they do get a lot of people donating their entry fee in the hope of getting a place (which I think statistically makes you more likely to NOT get a place so that they can make more money).
BK - I honestly don't think they will cap the price whilst there is such a demand for places.
As far as I know, Brighton Marathon put their prices up for this year's race to around £70, which is the highest price I know of for a marathon in this country - and people are paying it. I won't pay that much for 26.2, but it's down to the individual.