Either way (fiancee or marriage) you will have to apply for an additional visa 2 years after arriving in the UK which is the expensive one (I think currently it is £820). Taking the marriage route saves you having to apply for FLR which is cheapest visa...
If the rules change with the spouse route you will have to apply for 3 visas (4 with the fiancee route) and I've no ideal what the cost will be.