Modelling Asset Rentals

I am trying to create a model of an asset rental process, simplifying this slightly I have 2 dimensions and 2 facts, however my facts are at different grains but I need to compare them.

I have an asset dimension

asset_dim with asset_id, asset_number, asset description

a fiscal period dimension

period_dim with period_id, year, period , YYYYPP

and asset facts

asset_fact with asset_fact_id, asset_fk, period_fk , purchase_cost,hire_revenue

hire revenue changes by period but purchase cost does not. For return on investment calculations I need to see hire revenue as a percentage or purchase cost for either a period, year to date or life to date.

I have tried modelling so the purchase cost is 0 for all rows in asset_fact have a zero purchase cost except for the period when the asset was purchased but I could be reporting on hire revenue out side that time frame, I also tried modelling so that every row contains the purchase cost but then for a year to calculation I end up with upto 12 x the purchase cost.

Can anyone advise on a way to model this scenario.

Many thanks in advance for your help