<p>Oops I hit return too quickly.</p>
<p>So I am trying to figure out the math of how much to borrow on a home purchase. Hypothetical: Say I buy a $500K house and I have $500K. I am just trying to figure out if it makes sense to borrow.</p>
<p>One option is to pay the house in full and be done with it.</p>
<p>Another option is to borrow some money and invest it. Say I can borrow $100K at 5% interest (yes I know that is below market, but bear with me). I won't have to pay income taxes on the interest payments. The monthly payments would be something like $525. Most of that amount would be interest, let's say $500. Assuming I am in a 35% marginal tax bracket, that saves me 500 x 12 x .35 = $2100 in income taxes. I can take that $100K and invest it with Fidelity in a money market fund that pays 5%. Of course, I have to pay 35% of what I make in taxes. So I make about $3250 per year from this $100K I borrowed (100,000 x .05 x .65 = $3250).</p>
<p>So at the end of the year, I have to pay $6300 in principal and interest (525 x 12). But of that, I get the benefit of the principal payments ($300). I also save about $2100 in income taxes, and I make about $3250 in interest from the Fidelity investment, which totals about $5650. So it cost me about $500 for the year to borrow $100K. </p>
<p>One flaw in all of these assumptions is that I can get a loan for 5%. By the same token, assuming a 5% return on investment from Fidelity is pretty conservative. I also simplified some of the calculations. But what did I miss here?</p>
<p>Does it make sense to borrow if you don't have to?</p>