A custom bike is expensive whether it is made of steel or Titanium. I opted for steel when I bought my custom touring bike, but I could have ordered Titanium for an additional $1100 or so. But that would have pushed the price into the upper reaches of my financial stratosphere. It was too much for my pocketbook to handle!
If money were no object, I probably would have gone for Titanium. The idea of a strong, lightweight and rustproof frame is very tempting. But money is an issue, and there is nothing shoddy about my steel bike. The frame is strong, lightweight (compared to a Long Haul Trucker, but not to a carbon racer!) and, until the paint gets damaged, it is rustproof.
There is no guarantee that a Titanium frame will last longer than steel. Bikes do get stolen and banged up in accidents. A frame made of an exotic metal does not immunize the bicycle against thieves and potholes. All bikes need care. Any decent bicycle that is well maintained should last 20 or more years, and perhaps that's long enough. NOTHING is forever, not even the universe itself!