That doesn't mean the job won't be expensive.
I'll back him over you any day when it comes to executing big construction projects.
As would I. Nevertheless, rebuilding Alcatraz would be expensive. The concrete would have to be mixed there. The aggregate would come from a long way away. The loading and unloading would be subject to conditions and a bigger dock would have to be built. The diesel and fresh water would have to come in by barge, as would the mixing equipment. The tidal currents there are about 4kt, and the winds are often up to 35mph. So anchoring said barges would be difficult and operations limited by weather. All the concrete would have to be pumped at least 150ft uphill. NONE of that is cheap compared to building a prison somewhere else.
To Trump's point, I suspect he is showing his age, as there certainly was a mystique about Alcatraz as inviolable up until the breakout of 1961. But the place certainly does not carry that aura today. Frankly, I like the idea of putting a new prison there, but it wouldn't be cheap.