Is it possible to calculate the total value of the products assigned to a lead?

No - but, if you have a SMALL number of products with FIXED VALUES you can cheat and create custom fields in the leads that align to products -
create 'check box' fields for each product (e.g. product 1, product 2, product 3)
and then create a formula field - currency, 2 decimals and use this formula
If(${Leads.product 1},150,0)+If(${Leads.product 2},10,0)+If(${Leads.product 3},311,0)

this'll calculate a total for the boxes checked -

then if you also have those fields in your potential, you can map the products fields - but you have to create the formula field manually in the potentials and change the formula to use the ..."Potentials.product 1", etc - and when the check boxes are moved over during the convert process the Potentials calculated field will calculate and populate --

now your sales person only needs to take that value and put it into the "Amount" field to have the total go into the revenue stream and statistics .. a bit long winded, and limited, but it does work