This is a very important question. If the incentive’s too low you’ll have an issue not only in raising respondent interest to the market research project but also ensuring a good respondent turnout/participation. On the other hand if you pay too much you’ll be unnecessarily eating into your project budget.
The best way to determine the correct incentive is to put yourself in the shoes of the respondent. What would it take for you to attend a focus group or to take part in an online survey? Particularly in respect of focus groups, consider what incentive would ensure your attendance. Would you still attend if a better opportunity presented itself - perhaps your friends have asked you out for drink? What if it was raining and you were coming by bus or train? There are many obstacles that can prevent a respondent’s attendance, but if the incentive’s good enough this should be enough to overcome them (in addition to the effective management of the recruitment process by the highly specialised MRFGR respondent recruitment team).