How much should a new introducing broker-dealer allocate to a monthly retainer for legal/compliance & advice/services?

There's no way to really answer this question without knowing details about the work that you would want done by a lawyer or a compliance consultant.

Many things can be done in-house if you have competent and experienced folks in your compliance positions, but sometimes they need to be supplemented outside consultants and counsel. In addition, other things such as regulatory requests, exam responses and arbitration claims may increase the need for legal services and therefore drive up costs. I'd be happy to chat with you about your situation and possibly recommend some compliance consultants that might be able to assist your firm. You can also find valuable information on legal and compliance issues relating to broker-dealers at my blog at www.bdlawblog.com.