You're not really giving us much to go on other than size and weight. Do you ride trail or track? What surface, etc....
My wife is 5'4" also and her 150f fits her just fine. The 150f is 236 lbs wet, so you'd have to determine if it's too heavy for your son. My wife struggles to pick hers up, but to her credit she always tries first and usually gets it before asking for assistance.
The 2 strokers will probably be lighter, and maybe taller if they're true motocross bikes and will likely have more instantaneous power and speed than the 150f. The 150f is smooth as silk and feels light and maneuverable on the trails in comparison to my 230f.
You can use
www.craigzoom.com to look around the area for what bikes are going for. I'm assuming that craigzoom works in canada, but don't know that for sure.
As for what to pay for a bike, I like to use Kelly Blue book (
www.kbb.com) and look at the retail and trade-in values and pay something close to the middle of the 2 values depending upon condition of the bike. I find that a lot of people are asking a lot more $ than kbb says the bikes are worth. Again, I'm assuming that Kelly Blue Book is relevant in Canada.
Hope this helps just a little...