I just got done with this myself, having been posted to Gagetown from Pet this past spring/summer.
I used every penny of my interest buydown, both my entire monthly salary, and $1,500 left over in my custom envelope.
All banks will let you use the personalized (monthly rate), but you can only use Custom funds if you go with Home Loans Canada (CIBC).
The thing about the buy down is that the cheaper your house, the higher the rate buy down. For ex, if you bought a million dollar home, your $6,000 might only buy down your 4% loan to a 3.95% loan...but if you bought a $100,000 house it might buy your 4% down to 3%.
On my $222k house in Oromocto West, I was able to buy down my locked in 3.75% to 3%, and then with my extra $1,500 it ended up at 2.8%. Highly recommended savings yes, but I had nothing but bad luck with HLC/CIBC at the beginning.
They completely misplaced my funds for a month (until I prompted them), because my first two payments came out showing no buy down at all. After I lost my mind in rage I had the receipts from the lawyer and IRP showing that the money had been spent, and then CIBC finally saying "whoops, we accidentally put the money in the wrong spot".
Make sure you track your pennies!
If there's one tip I can give you, it's buy a trailer! Even if you have a Honda Civic, spend the $1,000 on a trailer with the wiring harness. Here are three reasons why:
1 - When the movers pack your house, they won't pack anything liquid. This means not just booze, but everything from shampoo to tanning oil. If you add up all these things in your house, they are likely several hundred dollars. Your trunk will be full of luggage and valuables, so put the piquids on the trailer!
2 - Every time you move, you will get 50% more mileage if you have a trailer. So say you would get $1,000 for gas and wear and tear, you'd now get $1,500. Yes you actually will spend some more on gas money, but certainly not 50% more for a little 8x5' flat trailer.
3 - Say you have something like a motorcycle or snowmobile that you would have to pay more for out of your custom envelope ($300-$500) for the movers to ship. You know where I'm going with this: put it on the trailer!! Save that $500 in your custom envelope for more buy down $$.
So in one move, for the cost of $1,000 to buy it, and $200 more in gas, you've saving $500 if you have a bike, getting $500 more free money, and saving several hundred in alcohol / other liquids. So if you have a motorcycle, in one move you have a free trailer. If you have no bike, it will take you a couple moves to get your free trailer.