I'm much cheaper than that... I did 2" foam board and then those kind of cans to seal around them.

Foam board was cut into pieces roughly and inch shy of the cavity dimensions... E.g. 14*5 for a 15x6" opening. Once the foam adhesive set (e.g. following weekend) I sprayed around the edges to seal it in.

I had to get a 30" gun adapter to do it which requires cans that can be attached to a gun. The ones that can't may not have the reach to get you into the cavity.

If you have full access to the cavity, that dap kit someone else sent would be my go-to... I didn't have the immediate access or will to do it all in one go... There were unmentioned complications in my way requiring lots of prep work.

E: Code likely requires a fire block between the living space and the foam. Afaik drywall, rockwool, among others count.