The capital required to construct a tidal barrage has been the significant stumbling block too. It is not an attractive proposition to an investor due to long payback periods. This problem could be solved by government funding or large organisations getting involved with tidal power.
In terms of long term costs, once the construction of the barrage is complete, there are very low maintenance and operating costs and the turbines only need replacing once around every 30 years. The life of the plant is indefinite and for its entire life it will receive free fuel from the tide.
Few tidal barrages have been constructed. The largest tidal power station in the world (and the only one in Europe) is in the Rance estuary in northern France. La Rance was completed in 1966 and has operated reliably ever since. So too has the barrage at The Bay of Fundy in Canada - though this had an adverse effect on Marine life.
There have been plans for a "Severn Barrage" from Brean Down in Somerset to Lavernock Point in Wales. Every now and again the idea gets proposed, but nothing has been built yet. It could have over 200 large turbines, and provide over 8,000 Megawatts of power (over 12 nuclear power station's worth).
It would take 7 years to build, and could provide 7% of the energy needs for England and Wales. There would be a number of benefits, including protecting a large stretch of coastline against damage from high storm tides, and providing a ready-made road bridge.
However, the drastic changes to the currents in the estuary could have huge effects on the ecosystem so it is unlikely ever to be built due to the major environmental impact that it would cause.
A major drawback of tidal power stations is that they can only generate when the tide is flowing in or out - in other words, only for 10 hours each day. However, tides are totally predictable, so we can plan to have other power stations generating at those times when the tidal station is out of action.