If you are in the USA for 7 days by car, you get $750 free purchases. They will than charge you 5%GST on anything over 750 that you declare.
But the CDN dollar is very good and if you bought $3000 USD of stuff, it would cost you about $3200 + 122.50 =3322.50 CDN for 3000USD.
If flying and it is Vegas, than you are hit big time with weight restrictions.
And they can charge you for new stuff installed. You have to PROVE that the new stuff was on before you went down.
Good luck with that if you get a nasty customs officer. Of course, if you declare it, than no problems, you just pay as above.