Cheap. Well, there's nothing works quite so well as old fire hose, like Bill says. My Grandfather used it on the dock as long as I can remember. And it's free.
On my 8' pram with mahogany rub rails, Granddad went and bought some white garden hose, and we screwed that into the rub rails with oval screws and countersink washers. Not nearly as big as fire hose. I think it still exposed some mahogany to barnacles from time to time, but you could also see that there was pretty brightwork underneath. I don't think garden hose would work for most aluminum rowboat gunwales I've seen...
But if you wanted to pay for something... maybe you could pick up some pool noodles at the local dollar store, slit them, and fit them over the gunwales. Mind you, they won't last long in the sun, and they might not work too well, but they could be cheap.
I did put pool noodles on my lifelines to be back cushions when sitting on the cockpit coaming and leaning back. I do not expect them to last much more than a season, what with them being a polyethylene foam, and UV taking a toll on PE, but for $2, I can swap out frequently if need be. Cheaper than buying Sunbrella covered cushions...
I found a $3 pool noodle at the local Dollar General store, that had a significantly larger hole down the center. I cut a slit, and I now put my Ruddercraft straight wood tiller into it. After putting 8 or 9 coats of Le Tonks varnish on it this winter, I don't want it bouncing around the bottom of the boat when I'm trailering. It must have had a 3" inside diameter hole. As opposed to many pool noodles that are only maybe 1" ID...
But, no, what you really want to do is to get some old firehose for free.
Brian