How close do you want/need the boat to be to Boca? Your best bet is finding someone with an unused lift near where you want to be: http://westpalmbeach.craigslist.org/boa and http://fortlauderdale.craigslist.org/boa
There's a "residents only" ramp at the ICW bridge on Palmetto Park Rd; big money for non-residents, not sure if you can get away with sister getting the resident pass.
A little south is a free ramp, Pioneer Park in Deerfield Beach. There are a few rack facilities near there on the same canal. Only one I know off the top of my head is Marina One. The other two are further up the canal, but I'm sure waaay cheaper if they have space. Marine One probably won't, but those other two "might" accomodate keeping the trailer, or even offer on-trailer storage; just a thought...
I keep my boat in a storage yard in north Pompano. I can PM contact info if you want to see if they have room.