There's a lot of good information in this thread already so I'll just share why I'm using heat mats and lights and what I purchased 3 years ago and have served me well.
Total cost was under $100.00.
My backyard garden space is just over 1000 sq feet (about 93 sq meters). I wanted to reduce my expenses by growing seedlings for transplant instead of buying them at a local greenhouse/nursery. I also wanted to grow varieties that most nurseries don't carry around here. I don't have enough natural light in the room where I grow seedlings and needed to supplement with LEDs. The room is also significantly colder than the rest of my house and so heat mats were a necessity. I normally plant 70-80 peppers plants each year. I sow seed into cell packs indoors in early March and target early to mid-May for transplant (central VA, Zone 7a).
There may be better or less expensive products available but I've used these very successfully 3 years in a row and I would buy them again.