Check out Odoo. Been using it for 3 years and has transformed by business. Does much more than CRM, we use it for sales, purchasing, accounts, HR and project management. But there are lots of other modules that can be added and customised. You can also have their eCommerce module integrated, not sure it would be entirely relevant to consulting but I've seen firms monetise their expertise with sessions, videos and programmes you can buy.
Would be very cheap to get started and you effectively pay for what you use so CRM would be cheap, and would give you the scope to gradually build it into a full scale ERP system to rival SAP when you're turning over tens of millions!
Try Zoho CRM