There are cheaper ways, but it's going to end up being more hassle than the cost savings are worth IMO.
A 30a/240v NO relay with a 120v coil could provide power to the lights, attach a 10a/120v NC relay to the coil of the 30a with an attic fan thermo attached inline to the coil of the 10a relay with a timer on the end of that trigger cable. That won't take care of the 15 minute delay though, so you'd need to integrate a timer circuit into the line after the thermo.
See what I mean about hassle
BTW any electricians out there, please verify the above info for errors...
MTF: I read your idea from someone else thread earlier and I agree with you..I have a Hot/COLD thermostat I used to use on my Dayton...then I started to use 1000w's and It was always too hot..lol...so its just been sitting around gathering dust..
im almost 100% sure im going to go with the setup in that diagram...i was just looking for someone to chime in with a "prebuilt" package option...btw I run hortilux bulbs so the 15min delay would be a life saver in $$$ and abuse to my bulbs..