Cosworth is as cheap as you can get the parts then someone to convert and fit maybe £1000+

Gotech is around a £1000 and jamsport fit and map it for you and the results have come back excellent.

Megasquirt is around £200 for the kit and is made for the fiesta so it is pretty straight forward to install if you have some experience.
Injectors vary depending on your choose standard cosworth yellow, light green, saab turbo etc etc.....
Then a laptop these can be bought on ebay for as little as £99 or like i'm doing next winter is ..... there is a sparky at my work is going to build a touch screen with the software installed that can be built into the dash like a normal monitor and can be run on line at all times saves lugging a laptop around every where.

There are many options best speak to a tuner of your options to your budget, or for more info try asking on the escort bit of this site there loads of lads on there that have gone for aftermarket management.