2 ideas.
1) Park at Luton Airport somewhere (about �20 for 3 days). Take the shuttle to the airport, then the free shuttle to Luton Parkway station. Its about �20 return to St. Pancras. (We almost did that this year, as we were going to fly back from Barcelona, but plans have changed).
2) Drive to the new station at Ebbsfleet (nr Dartford) which has massive car parks. I've tried looking on line for prices there, but the Ebbsfleet website is still under construction, so you will have to phone Eurostar and ask them.