"how much maintenance really goes into these things? "
The preamp tubes should last for a very long time, unless they get physically damaged, or the failure of another component causes trouble. The power tubes life expectancy is dependant on factors like how much stress the circuit design puts on them, and how hard you play. Cranking the amp full-pin all the time will wear them out more quickly. You will eventually learn to recognise the tone loss caused by weakening power tubes, just like knowing when the guitar strings need to be replaced.
"how careful do i have to be carrying them?"
Don't move an amp while its running, or until the tube filaments have had time to cool off after shutting down. A tube amp is not super-fragile, but bashing it around is bad policy.
"how much does biasing cost (I assume it's about once every 2 years for the power tubes) "
It depends on how hard you run the amp. If the tone is kinda flat, its time to get it checked out. Some amp designs do not require adjustment. Check out whether your amp has 'self-bias', 'cathode bias' or 'fixed bias'. Pricing varies, but for a simple job, a half-hour should do it. If the bias circuit is not variable it can take longer, and will cost more. My old Ampegs, for instance were adjusted by physically replacing some components to get the right voltage. A common mod is to install a trimpot to make the voltage easy to adjust. So the first visit would cost extra for modifying the circuit, but after that, the bias adjustment would be quick, easy, and less expensive.
"and how much do Powertubes cost?"
How much do you want to spend? You can get JJ EL84's for about $20. You can get New-Old-Stock Mullard EL34's for about $1000.
"What is biasing?"
Its equivalent to setting the idle speed of a motor. As the tube amplifies an AC audio signal, the voltages swing up and down from a 'starting point' which is set by adjusting the bias. Some amplifier designs called 'self-bias' or 'cathode bias' are set up in such a way that this bias changes itself as the tube ages. No need to do anything but Plug'n'Play. All preamp tubes use this kind of circuit, and a lot of lower-powered amps use this on the output stage as well. A circuit design called 'fixed bias' has a circuit that does not self-adjust (the setting is 'fixed'). These amps need to be adjusted by a tech periodically to keep the power tubes happy.
