£500 isn't that bad, and only needs doing every 10 years or 125k miles I think on these.
Expect it to be around £600, as you'd do the Clutch at the same time whilst it's all apart. (I just had the Clutch done on my MK7 Fiesta - non ST and paid £700 which was on the higher side of quotes I had)
Even with both of those costs though, it's a case of whatever else you buy (especially older things like e92's) could easily need bills just as big before long.
Have you looked at Mountune or similar for your Mk7 ST - 215PS with only a self installable map, and an induction kit?
The MT215 mTune comes preloaded with the correct ECU calibration engineered and developed to add maximum reliable performance to your Mk7 Fiesta ST, when equipped with the mountune induction kit. The kit delivers 215PS and 320Nm when combined with the correct hardware, enough to transform the...
www.mountunestore.com