Well, Gemma Ward made about $3 million last year. She was the highest earning teenager in Austrailia.
I know Heather is not in the same league as Gemma (since she and Daria are basically #1 and #2 in modeling) but she is not an unknown model. She's fairly well known, gets big shows, has a few commercial campaigns (Revlon, VS, NM), which pays more than high fashion, doesn't it? And she probably has other commercial campaigns we don't know about ... remember the makeup campaign, called RMK or something, from Asia? I bet they go wild over Heather's look in Asia.
And lastly... Heather only has to support herself, which I think definitely helps her save money. She doesn't need to take care of a family or someone else. She's home in Calgary a lot, so I expect the food/utility bills aren't as high when she's living at home, which does factor in considering she lives in NY. And she doesn't wear only designer clothes that would cost a lot of money. In her candids, I usually see her wearing stuff from Aritzia, which is a lot cheaper than the likes of Marc Jacobs and D&G.
If she made $300 000 from the Marcus shoot, her agency only takes about 20% commission, don't they? As for taxes... people aren't always honest about them. I know a model who was given some handbags as payment when she was in Asia and at the airport, she said they were fake instead of real so she didn't have to pay as much taxes. Not saying Heather does that, but you never know...