I booked jasper only 2 days prior to my arrival 2 days ago and got a full service site at the Whistler CG, must of got lucky! Or maybe it's because I phoned up and paid the $13.50 booking fee, because when I checked online ($11.00 booking fee) there was nothing available for my 30' TT, only rustic sites, which where still available apon my arrival. Also be aware that you will have to pay another $19.60 if you have more than 3 people, slightly cheaper for 2 and I think $9.80 for 1 person for every night you stay in the national park. A fire will also cost another $8.80. It all adds up! And to top it off, I took my 5 y.o. Daughter to the toilet where I had purchased 2 latte's and it cost a quarter. I'm sure that's the French influence. If you've been to Paris you'll know what I mean. :)
My nights stay at jasper $77.70 ouch! Inc. Full serviced site $35.80. The only avoidable cost is the fire. The sad thing is I did it all again in Banff.
Now if you're staying more than 7 nights you can get a national park pass valid for one year for $136. If money isn't an issue, go for it, otherwise!
Go to the US it's a lot cheaper. Glacier NP for example.