In reply to pasbury:
It's been a while since I used to regularly get out in Snowdonia. Then I used to buy the season permit for its car parks.
I have just looked this up to see how much it is now and it's either £100 for all apart from Pen Y Pass Snowdonia National Park owned Pay and display parks or £250 including Pen Y Pass.
http://www.eryri-npa.gov.uk/visiting/local-information/car-parking
A little bit more than I think the £45/50 it used to be!
Basically 20 visits to any pay and display covers your cost for the £100 if your staying longer than 4 hours.
This doesn't really help in this instance but as you were staying for a few days if it was a regular occurrence the above may be considered if it was a shared cost with others (e.g them contributing to parking etc).