Do they want the playing board? Or do they want a Monopoly themed cake? With the themed cake you could make a 2-3 tiered cake and then place Chance/Community Chest cards, money, playing pieces, and properties around the tiers.
For the playing board, would they want the whole set up? or would a section suffice (just doing the boardwalk/park place section or another color section). You could decorate the colored section with houses/hotels and one or two different player pieces. Maybe have some money drifting down the sides and some dice lying on top also.
if they want the whole board-wow! that's going to be detailed! I can't even imagine how long that would take you-if that's the case, definately price higher for the amount of detail/time involved.
maybe someone else will have some tips/ideas for you- good luck!