It is zero! This is because you are NOT covered by the NHS while on a fiancé visa. You will need to take out travel insurance to cover you until you are able to switch to FLR(M) at which time you will pay a £500 levy.
If you do NOT have insurance, you will be charge 150% of the cost of any treatment received. With insurance, it's 100% of the cost of treatment received. An outstanding bill of £500 will result in an automatic refusal of your FLR(M) visa.
Nothing wrong with receiving treatment while on your fiancé visa. Just INSIST on a bill and receipt for payment.
Current fee for the fiancé visa is £1,195. This will be billed in GBP to the card you use. So if your credit card company or bank charges a foreign transaction fee, you'll need to include that as well. If you want to do priority processing, it's an additional £450. Biometrics is included in the cost.