I think most of you are missing a point. It is fun to build something from scratch. It's like playing with Lego's, except that the parts usually need more or less modification to fit properly. Yes, it takes time and money to do so, but considering the many hours you are occupied with an interesting process of creating something unique, are well spend. If you (like me) like to challenge yourself and keep yourself entertained by your hands, go for it, even if you know you are likely to buy parts that in the end are wrong for the project.
I do not agree with using a donor gun is the cheapest way to make a custom build. In the end you will end up with parts enough for 2 separate guns. I made an M4-SD model and there are only 4 parts left of the original gun (springs included in this count), everything else has been changed. I could have saved US$270 on that build by not having used a donor gun.
The price of a build are as others write correlated to the parts used. I'll give you an example of my last build that was done from scratch. I'll keep the price in US$ as that is what the Hong Kong stores charge you in. But do note that you are likely to pay shipping from 2-3 different stores if you need your parts fast or need something special that not all stores stocks. Also you will need to add all fees to the equation as I do not know the Canadian system. I Denmark where I come from, we are charged around an additional 28.4% on top of the price including shipping and I really hope the Canadian customs lets you guys of cheaper than this.
Anyway here is an example of a build based on my latest project that I have adapted to a team mate that wanted some similar.
Front end kit: 130$
Ace stock:47$
Metal body: 212.50$
Grip: 30$
Trigger guard: 25$
Compleate gearbox: 220$
Motor: 55$
Inner barrel: 35$
Hop-up unit 30$
Hop-up rubber 8$
Accessories set 37$
Battery pack (9.6v on ELITE1500 2/3A cells (special made to order)): 30$
MP-Jet 3.5mm connectors: 4$
1.5mm^2 wires: 10$
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total: 873.50$
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There are always things to fiddle around with when building from scratch, just finding the right parts alone take hours. Modifying parts to go together are a nightmare and troubleshooting takes forever. You can always makes things more complicated by building the gearbox from scratch etc.
If you really have the guts to take on a 3 dimensional puzzle that does more than looking nice, go ahead. But be prepared to accept that you make expensive mistakes, from time to time. You will learn from it and you have the opputunity to get a true one of a kind gun.