But with a spouse visa ($800 approx) I can land in UK and start to work right away and have up to two years before I need to get a FLR ($680 approx). So the benefit is that we'd only pay the $800 to get me to the UK instead of $1460? Is this correct?
Not quite - the spousal visa is essentially the same as the FLR, but it has been granted from outside the UK (FLR is the name given to the spousal visa when applied for from within the UK). What you apply for after 2 years is ILR/permanent residence (at the moment anyway, but this is likely to change). Whichever way you go about it (Fiance visa -> FLR(M) or spousal visa directly), you will still have to apply for ILR 2 years after you start the FLR/spousal visa.
So as of the current rules and fees you can either pay for fiance visa ($800), spousal visa/FLR ($680) and then ILR after 2 years on FLR ($1,200 by post, $1,500 in person), or you can pay spousal visa ($800) and then ILR after 2 years ($1,200 by post, $1,500 in person).