Where does the money for this kind of hobby come from?
Simple. Make gradual purchases or make a large purchase and don't buy anything else until you complete all that stuff. Impulse buying is what will get you over-time. Know what you want and control spending like one would do in most things. $400 is quite a bit unless you're spending that every single month, you should have more than enough from any typical job to make a few purchases every few months.
I know Warhammer is expensive, but so are hobbies like computer-building and video gaming and unlike certain non-productive related activities, they don't disappear after you're done using them. Furthermore the time you spend in the modelling, detailing, painting and playing will generally make up for that cost in terms of enjoyment or achievement. I've only halted my 40k stuff because I'm so busy and I keep procrastinating.
Ah, I wish I had a large enough table to accommodate my computer and miniatures. Just be watching streams while painting.
Things to do if you're starting nowadays in terms of the models themselves since the Captain covered the basics:
1. Start Collecting Boxes. You generally get a free unit in these boxes and many of these contain the core essentials to any army.
2. Discount Retailers.
3. Split large purchases. GW's been recently releasing those large box sets, often with two armies. Want one half but not another? Try to split it or sell off the stuff you don't need.
3a. Try to pick up the rules from said box set if it comes with any.
4. Kit-Bash stuff. Sure there are plenty of online retailers, but nowadays you get so many bits and leftovers that often if you buy something like a basic unit, you can often modify them to be the elites/special troops. (SM are probably the simplest for this)
5. Sales at said discount retailers. Furthermore some events, like Gottacon that I used to go to while I studied in Victoria will bring their prices down when the event is nearing the end. Great time to pick up things you wanted. It's where I got my 20+ Raptors from.