it's amazing how condesending rich people are about boating lol..
no one on this site is rich. we just know that economy and boating are a dichotomy. we also know that cutting your power in half usually means you burn more fuel.
however to put it this way, if you routinely used more than 1/4 throttle of your 110hp, then you wont save any noticeable money with the 60hp.
your 2-stroke burned about 0.5# of fuel per hp per hour.
the 4-stroke will burn about 0.4# of fuel per hp per hour.
each engine is different and the BSFC curves are not flat, however follow along
so if you routinely used 60hp to push your boat around with the 110hp, you will be at WOT all the time with the 60. and in one hour, you may save about 3/4 of a gallon of fuel, however you will beat the 60hp to death running it at WOT all the time.
at a fuel burn of 30hp, the 110 may actually burn less fuel than the 60hp, as the 110 BSFC curve may be better.
if your boat needs 50hp to get up on plane, then the 60hp may struggle most of the time if your loaded
since you left out so much detail, we cant really even help. the only detail we know is your current motor is a 110hp 1992 johnnyrude and your looking at a 60hp 4-stroke
what hull? a light hull means nothing if it has the hydrodynamic profile of a brick
what is it rated for? - many times you need to stay at 75% or higher of max rating or the hull is a dog
do you boat in wind?
do you boat in current?
is the boat loaded?