In general, you can expect different costing systems for BIM services as most of the consultants and companies have their own ways for pricing and can be categorized as:
Price per square area method
Per hour method
Lump sum method
Each scenario has its own pros and cons, but the attributes discussed above must be properly addressed out no matter what the pricing method is. Cost per square method is usually fixed while per hour method has a variation of 10-15% from the baseline hours used while estimating. Still, it is good method when you realized that it’s a balanced risk distribution where BIM service provider feels more comfortable as he will be compensated for extra work hours.
Lump sum method can be used in combination with above two scenarios and applied when limited information on project is available and client demands the rough estimate. Still, it’s the least recommended method owing to higher risk ratio unless until you are confident enough about the expertise of the BIM consultant.