First, welcome to YorkieTalk.
Putting up with heat isn't too bad in most cases but there are several health benefits to spaying. You will eliminate her chances of getting pyometra. A dog who is spayed before her first heat has less than a .5% chance of getting mammary cancer. If she is spayed after her first and before her second heat the chances go up to 8%. If she is spayed anytime after her second heat the chances of mammary cancer go up to around 26%.
It is best to do health testing on the male on female before breeding so that there is less of a chance of them passing on certain diseases. Bile acids testing is very important and a blood panel. Testing for thyroid problems, luxating patella, hips problems and eye and ear problems are also a good thing to do. This could get expensive. Along with all of this, their lines should be clear of certain diseases, so you will need to know their pedigrees.
When breeding, the female should stay with the male. So your girl would need to leave home for a few days. If she has trouble whelping, a c-section will be between $300-$2250ish. If she gets eclampsia, the pups will have to be bottle fed all through the night every night. They also need supervision for awhile after birth. You'll need to pay for vaccines, dewclaw removal, tail docking, health checks for pups, prenatal checks for mom, xray and maybe ultrasound, etc. Before breeding, it is good to educate yourself on the whelping process so that you can assist if necessary.
In rare cases, the mother can be lost during all of this...
If you decide breeding is for you, it is a good idea to go to shows in your area, talk to breeders and try to find a mentor.
Some people love it and some think it is just too heartbreaking and stressful.