I have mixed feelings about this one. I think up to a point when a child is old enough to do one or two chores then they should do so - unpaid - as part and parcel of living in a house and as part of their family. I don't feel there should be a price attached to everything you do.
But I also do think that good behaviour should be acknowledged (and on occasion rewarded) too. I always remember to thank my children for helping out and reinforce the idea that their help helps us all as a family and that it's considerate, grown-up etc. We give them treats anyway, not as payment as such, but if there has been especially bad behaviour that can stop and it's pointed out why.
Older dd(7) has just started to have pocket money. Dh gives her a little extra if she has been especially good or done well, but jobs etc don't have a tariff as such. I wouldn't want it to be that cut and dried - it seems a bit mercenary to me.
I suppose we're trying to strike a happy medium on this really because I think it's nice to do things just because you know they need doing and want to help out rather than expecting something in return all the time. If you are rewarded too then that's a nice extra iyswim.