Depends on what you already have.
If you currently have single DIN, then you will need the double DIN cage, double DIN heater controls (or cut yours down to fit), RNSE, wiring loom and coding.
If you don't have any of those, then you need to budget for all of the above, plus fitting.
No one can give you a definate price unless they already know what you currently have/don't have as you've not stated anything on your post.
My best guess, based on Happy Days pricing is close to £1000 if you don't have all the above.