There are 2 types.
Cheapest is the finger prick type that uses test strips. The monitor itself can be cheap but watch you chose one were the strips aren't too expensive. At first , while you learn, you will get though a lot of strips, perhaps 8-10 a day
@Rachox has some info on what and where to buy.
The second sort are the continuous monitors where a sensor takes your bg through a fine needle being stuck to your arm. Results feed to your phone. Each sensor lasts 10-14 days depending on make and costs about £50 a week.