if you are talking about sealing the attic vents(soffit/gable/ridge) then DON'T. you need them open to allow the attic to breathe. close them up and you get humidity issues.

They are air sealing between the sealing and attic floor. While you are at it, get a blower door test done before/after - if the house is too "tight" you may need an ERV or HRV.

I am not sure about the sealing costs, but here in PA the cost for spray foam is $1-2 per square foot for 1" thick - would be more for an attic since removing insulation to foam underneath is not fun. The air sealing cost may be less, since they will just foam drywall gaps, edges, intersections and plumbing penetrations.

The attic air sealing will be very worthwhile. If your basement is unfinished do the band joist also - a huge sorce of leakage.