If you think $80 a quad is alot for power tubes, you 'aint seen nothin' yet
Which you should change out first will ultimately depend upon the current amount of hours on your tubes--if your power tubes are getting old, you might get the most sonic benefit from replacing them first. All things equal, I think preamp tubes make more of a difference, assuming equal quality tubes throughout.
I have just concluded a nice set of tests with 12AX7s:
New Production:
EH 12AX7
Ei Elite 12AX7
Groove Tube "Mullard" 12AX7M
Sovtek 5751
NOS:
Telefunken Smoothplate
Mullard Shortplate
Mullard Longplate
TungSol 12AX7
What type of sound are you looking for? I can give general impressions, based upon my tests (although keep in mind all gear is different and tubes may well sound radically different depending upon the circuit, I think generalities are OK).