Depends. Sealants are a terrific service and you are making a great investment in your (or you child's?) dental health. Like most things you purchase, it depends on where you live. In the san francisco area, the average cost of a sealant is about $70+. Fillings can cost 5x as much so it is better to prevent decay than repair it!
Yes. You mean how much do they cost? Depends on whether you have insurance coverage, your age, your region, etc. Under $100.
Sealants. Sealants are wonderful preventative restorations that help prevent occlusal surfaces from developing decay. The price really depends on your local market. A cost of $40/sealant is reasonable on average.
Depends. Depends on the area of the country you are living in. Cost could vary between $25-$60 per tooth.