You can only extend if you haven't passed the Life in the UK test, otherwise you have to apply for ILR. In the long run you won't really be saving money though, because ILR will only go up in price, and you'll at that point have had to pay for 2 visas instead of 1. There is also a danger you could get caught out in new rules that would extend the required residency period for ILR. ILR gives you more rights as well, such as being able to apply for citizenship once you've been here 3 years, and being able to apply for benefits if you need them. There is no real benefit to extending FLR.
ETA: Also, if your FLR started in June, you can only apply for ILR 28 days before the expiry of that visa.