There are many ways to build these style of rifles. As other have said $1800-$2000 is about the low end. From what's Ive seen on the various FB groups I bet the average person spends about $2500 without an optic. Don't be afraid to buy stuff used as well, things like handguards, pistol grips, buttstocks, etc can often be found barely used or unused on the EE for a cheaper price. Brownells is also a good place to buy stuff, their brownells branded stuff is usually decent and cheap. The TNA unbranded stuff is cheap junk for the most part, others have called it 'airsoft' quality, stay away from it.
For a budget build I'd do the following
Go with the ATRS MS receiver set. I don't like my early MDI set, it has issues. My ATRS lower (still waiting on the upper) is much better quality than my MDI.
Upper Parts:
barrel: Ballistic Advantage 20" $230-$350 (Saskatoon Gunwerks)
Flash hider: A2 flash hider $8USD Brownells (a lot of guys will just give you one)
crush washer: Brownells $3USD
gas block: Brownells 0.75" $20USD
gas tube: Brownells: $11USD
gas tube roll pin: $1.29 (Nordicmarksman)
barrel nut: Colt $11 (Nordic Marksman)
Port Door Kit: DPMS $13 (Nordicmarksman)
Charging Handle: Brownells $15USD
Bolt Carrier Group: Brownells Nickel Boron $94USD
Handguard: Midwest industries G3 M-Lok 15" $170USD (Brownells)
$1182 CAD for the above, including $500 for the ATRS upper.
Lower Parts:
LPK without trigger or pistol grip: Brownells $25USD
Trigger: ALG Combat Trigger $110 (CTCS)
Pistol Grip: Something Magpul $25-$40 (Nordic Marksman)
Buffer tube Assembly: Brownells includes castle nut, receiver endplate, carbine spring & buffer $40USD
Buttstock: Magpul CTR $77 (Nordic Marksman)
$798 CAD for the above, including $500 for a ATRS lower.
I don't think I forgot anything. That comes to a total of $1980 CAD for everything. You will of course have to pay taxes & shipping on that stuff. Try and keep your buying to as few places as possible so you don't end up paying shipping more than a few times. Keep Brownells purchases to under $500USD, the paperwork (End User Statement) takes 20 minutes to fill out and email back. If ordering from brownells make sure you select US Postal for shipping.
That should be all you need to get started.