Cheapest option are the trading boats from Turbo, about eight to ten days and will cost you about $300 including bike. There is a captain Rafa mentioned on another post who goes regularly, but there are many, probably every four days or so.
Talk to people on the dock in Turbo, you might need to take a speed boat out of Colombia and meet them on the border as there are laws on passengers for this first section. Feel free to question me on this, i feel like the most qualified gringo on this area as i was living there for over two months at the end of 09´.
I would also encourage any rider to attempt the Gap again but certainly wouldn't take any responsibility for life or limb.