You normally pay whatever the current rate is when making voluntary NI contributions except that: "If you’re paying Class 2 contributions for the previous tax year or Class 3 contributions for the previous 2 tax years, you pay the original rate for those tax years"
https://www.gov.uk/voluntary-national-insurance-contributions/rates
The Class 2 rate for the tax year which is just ending is £3.05 per week. It remains unchanged for the forthcoming tax year (starting next week).
The Class 3 rate for the current year is £15.30 per week. It rises to £15.40 per week from next week.
Rates for the following tax year won't be announced until the beginning of February 2022 but are unlikely to be greatly higher than those for 2021/22.
Before making any voluntary NI contributions though, check whether it will actually benefit you. If you've already paid in enough to secure the maximum State Pension, there's no point in paying any more!
When I was approaching State Pension Age, I enquired about whether I was able to make voluntary contributions for certain 'gap' years in my NI records. The answer was 'Yes'. I then asked if, by making such payments, I'd actually be any better off with regard to the State Pension that I'd receive. The answer was 'No'. I suspect that you can guess what my decision was ;-)