Personally, i don't think it would make much money.  As swinks has pointed out, transport, damage and insurance would possibly cost more than the income generated.  I may be wrong.  It could be a good sideline through word of mouth, but you only need one machine to have a failure (parents leave kids alone for the night, they throw a big party and trash the machine, then do a runner) and it all turns bad.  It may be more worthwhile siting one somewhere you know the owner well, such as a corner store, take away joint or similar. Then if people enquire about it, the owner could pass your details and say you would consider short term hire.  But i really think the market is quite limited.

To be completely honest, you probably need to ask the question on a non pinball forum.  We all get excited about seeing a pinny out in the wild, so encourage others to do it. But if you are relying on it for income, you will get a less biased opinion from non pinheads.

As an aside - if you can find a place to site one that you are confident it is safe it may be worthwhile. As an indication, we have a tavern thingy 50m from our back fence (live in semi rural area) - http://www.melvilles.com.au/ it is only open Friday nights and Sunday lunch and has a country theme and is aimed at families - they always have a jumping castle, train rides and animals for the kids to pat.  I know the owner well (as a neighbour) and he was very keen for me to put a machine up there - his insurance covers damage.  In 6 weeks i made $150 with Roadshow, then swapped it over to Austin Powers last week, and on Sunday there were plenty of people giving it a workout.  Its not really there to make too much money, more to give exposure to the hobby - i've got it set on $1 for 4 balls.