Over the years I've tried a number of different solutions to getting my seed trays to the correct temperature for germination. The most expensive way is to keep the house heated all night. I've also tried using a fish tank heater in a pool of water under the tray. Which worked well, but is messy and potentially a huge mess. I've even bough a heating pad for putting under the seed tray, which also works but is expensive to obtain.
The easiest and cheapest solution I've found can be used by anyone with a standard hot water heater. Simply place the seed tray on the top of the hot water heater. Our furnace and hot water tank are in the same room and it stays toasty all night long without any additional cost. I used that approach this year and had my tomatoes all germinated in 4 days.
I should note that this will not work for seeds that require light to germinated, such as lettuce seeds. I don't have a full list of seeds that need light, but I do know that most don't.