I believe some improved account would be a great feature to add to WHMCS.
For server monthly costs (configured in the servers config):
Be able to add a per order additional cost. For example we sell server monitor through a partnership with a server uptime monitoring company. There is no monthly fee but every month there is a per service fee. So when it see 10 customers ordered that product assigned to that server than it would take 10 x (per service fee) + monthly fee to show for accounting.
it would also be great to add a per unit sold cost. For example if there is is a quantity config option field be able to add a cost per month per unit. This would be great for accounting for license services such as VoIP clients that we interface directly with via custom modules. This would be something you could setup during the configurable option setup to add additional costs that vary on the amount sold.