This doesn't take into account any work you have to do on the house, the notary and agency fees, and any taxes on the way. Your $300k worth of investment may very well cost you around $500K. If it's worth $800k later on, you earned $300k buy selling it 30 years later.
We can also add the cost of opportunity, since you can't move easily when you have a home to get a new, get closer to your children, follow you the love you just found, discover a foreign country...
And that's in the best case scenario where:
- You found a cheap good home.
- You actually end up liking leaving here.
- Nothing changes to the point you have to flea the place.
- No accident, no fire, no water pipe breaking, etc.
- You can ensure you can comfortably pay the mortgage for three decades.
That's... a lot to consider.
To give you an idea, here is what happen if you take the same down payment, and invest $100 a month and interests into the S&P 500 index for the 30 last years:
You end up with $11 159 316.
With no lock in. And no worries.
I think it's fair to say there is a difference of at least $100 between your rent and mortgage payment.