It's every ten minutes - quicker than going miles out of your way (unless you're Hippy).
As for the £3 - well, I can't imagine something spanning the river at that point, which would have to be either very high or openable or both, would be financially feasible without some kind of toll.
Sadly that's it - financially feasible - is the problem. It should be built because it would improve transport linksand connect up Rotherhithe, which is a bit of a dead area, but it won't. And no-one is going to pay to use a pedestrian bridge.
Yeah but that costs £3 a pop. And isn't that frequent. Or open at night.
RE Rotherhithe tunnel. Done it twice. Never again. I like my lungs.