Select datas from different years

[mongaro]

Dear All,
I tried to look in with the "search" function without success:
I have a db to manage projects.
Each single project has a subform that collects the activities and the employed resource in each activity ([Work]):

[Activity] [Resource] [StartDate] [EndDate] [Work] [TotResCost]
Production Line 1 01/01/2010 03/01/2010 15 1.500 €

Each resource has a specific hour cost ([ResCost] and the subform shows the total resource usage [TotResCost] multiplying [ResCost]*[Work] on each activity (record of the subform).
Now the request is sometimes to update the resource hour cost (yearly update) without loosing the "old" projects cost.
My idea was to create a table with the field [YearRes] and the related [ResCost] linked to each resource and find a way in the subform to filter the selection of the related [ResCost] with the year of the activity [StartDate].

i.e

if the [StartDate] is 01/01/2010 the resource's combo box shows only the 2010 [ResCost] and use it for the [TotResCost] calculation.

Any suggestion? I hope I explained clear the problem.
Thanks for your support.
Ciao

Marco