If you choose a shade midway between the highlights and the dark tone and do a root touch up, combing it through with just a few minutes to go to slightly tone the highlights down a bit you should blend everything in ok, Use a semi permanent colour designed for grey hair and don't leave it on the highlighted bits for longer than 3 mins. I know what it says on the packet but if you are careful andf use a neutral colour you should be ok, warmer beige tones often work better than ash unless you have really cool colouring
Been doing it with mine for years never had a major disaster
£150 sounds a bit steep I go to one of the better salons in Birmingham and mine only costs around £100 if I have 3 colours and a restyle from one of the design consultants might be worth looking for somewhere new,