Add the cost of a fuel pump, external fuel pumps are not cheap for injected cars unless you're prepared to make an old pump fit. Braided hose is very expensive, if you're interested in safety then it's best to run with the standard tank and fuel lines unless you're prepared to spend the money to do it properly. High pressure lines on a budget through the cockpit may be acceptable but it doesn't help safety at all. If you want to run MSA events and generally be safe then you need to have a firewall, properly fitted and sealed around the rollcage, it is a good days labour to fit a firewall well.
Approximate costs for fitting a fuel system:
Fuel cell - £300-400 (guessing you're talking a proper fuel cell not some homemade steel box)
Fuel pump - £100
In line filter - £15
Hoses/piping - £50
Hose fittings/connectors - £50
Wiring/switches/fuses and holders - £10
Aluminium for firewall - £40
Labour to get the job done by a professional would probably put the cost into four figures, and all for no real advantage. The standard tank will be better for performance anyway with the weight all very well packed under the car. In our race cars (on six figure budgets) we use the standard plastic tanks in conjunction with a fuel cell (for extra capacity), there's nothing wrong with them and in an accident I'd rather not have all the fuel in the car with me, given that the best fitting firewall is useless when the car changes shape.
It would be far wiser to ensure that the standard fuel tank is well connected, make new straps if you have any doubt of their condition, the mass of the fuel creates enormous force in an accident, a full large capcity endurance tank that is well mounted will deform the car if it is stopped fast enough. Check all the lines and if you've got time and money to burn renewing hoses is sensible.
