The cost to replace an existing fuse board with a full RCBO and surge protection will all depend on a few factors. How many fuses there currently are in the existing fuse board, if any work needs doing to the supply conductors and main earth and also if you have the correct earth binding to the gas and water in-let valves. If all of the above is ok and in place itís expected to be in the region of £550-£700 for up to 10 fuses. This cost may be different for other areas of the country as I am based in Berkshire. I normally always visit to provide a detailed quote as there are other aspects to consider that may affect the cost.
Hope that helps a bit!