Sonofnorway is correct about the standby mode, this is your best option. Do not turn it off, but I would recommend you put it in standby mode.
As for tube costs, the last I checked (8-10 months ago) BAT was charging $200 per tube for what they called NOS 6H30's. FWIW, the price is not going down, only up. I believe BAT was charging $100 per NOS 6H30 about 2-3 years ago, now it's $200 per tube. They claim these are from the 80's, and better than current manufacturer 6H30's. BAT was also selling current 6H30's for $40 to $50 per tube, check with them as I'm not positive here. I remember the $200 price because I had a VK-51SE and the thought of $1600 to re-tube it with NOS 6H30's left a lasting impression.
If you go through other tube vendors instead of BAT you could find 6H30's for as little as $20 each. So to answer your question, no it wouldn't cost you $500-$1000 CDN per year (assuming you're leaving it on 24/7) to keep you preamp running, you could spend as little as $80 a year.
It all depends on how 'good' of tubes you want to put in it, and if you can hear the difference between a $20 tube and a $200 tube. If you get hooked on the NOS 6H30's and leaving the preamp on 24/7, it could get pricey.
I would recommend leaving it in standby, then replacing the tubes every 3-4 years wouldn't be much of an issue, even if choosing the NOS from BAT.
John
