A colleague of mine paid £200 for an installation (excluding the cost of the consumer unit), which I thought was not bad.
But much might depend on how easy the replacement is – when I replaced mine a few years ago there was insufficient room for a swap out and I had to run extra lengths of cable to accommodate the new location. And in my current property, the regulations did not permit a new consumer unit to be located where it was within the kitchen.
I would suggest you get three reputable electricians to quote you. My advice is to have a modern consumer unit fitted with a split load. These units have two banks of circuit breakers each protected by an earth leakage trip. With one bank supplying power to the ground floor electrics and the other to the upstairs, should one bank develop a fault you will still have power within the property.
I think anyone with an old style fuse box should consider such an upgrade – with a modern consumer unit it is almost impossible to electrocute yourself, and will reduce the chances of an electrical fire.