It is shown that Henrys house was the house in which Shawn had grown up, so it's likely that Henry and his ex-wife (Madeleine) bought it together in the early 70s if not before, when prices would have been comparatively lower.
The average price of a home in the US in 1970 was $17,000 (adjusted for inflation that's around $120,000 today - 2022). Whereas the average cost today (2022) is over $290,000.
The average mortgage length in the US is 30 years. It is stated in the show that Henry and Madeleine split around 1994 and so there wouldn't have been much left for Henry to pay by himself at this point.
Additionally Henry is often shown doing a lot of work on the house himself, which would save on upkeep costs
