Mark is good , but is expensive. You do get what you pay for if you dont mind paying that. and maybe a runnout in Robins Red Temptress to plant a few more seeds.
...."expensive"? - Yes and no.
For example, I asked VWR, JMK, and APS, to submit costs for my Quaife diff - Supply and fit. There was only £50 difference on a total of about £1,500 between the most expensive (VWR) and the cheapest. But, yes, some mods are relatively expensive.
However, I challenge anyone to better VWR's standard of involvement and commitment. So IMO, VWR are excellent value-for-money, but modding mostly ain't cheap. It's Horses For Courses and depends what you are buying.