If you are married to a UK citizen, you can apply for citizenship as soon as you have the ILR instead of waiting 12 months... as long as you meet the residency requirements (no more than 270 days outside the UK in the previous 3 years, with no more than 90 of those days in the last 12 months before applying).
You can't do ILR and citizenship at the same time though, as you need to hold ILR first before you can apply for citizenship.
While the current price of ILR is £2,297, it is approved to rise to £3,250 in the next year or so, and may well go even higher by the time you qualify. The fees rise in April every year.
Citizenship is currently £1282, but didn't change much in price this last year, however the fees still increase in April every year.
In 2016/2017, ILR was £1,875 and citizenship was £1,236, then for 2017/2018 they are £2,297 and £1,282 respectively.
So, your timeline will be:
- apply for your FLR(M) renewal
- 28 days before you reach 5 years in the UK (which may well be 28 days before the expiry date of the FLR(M)), you can apply for ILR
- once ILR has been approved and issued, you can apply for citizenship, as long as you meet all the requirements.
- once citizenship has been approved you attend a citizenship ceremony within 3 months (I think)
- after that, you can apply for a UK passport
If you don't meet all the requirements for citizenship right away, you just need to wait until you do meet them.