Legos aren't cheap, but the Titanic is not $900. It's listed as $680 on the Lego site. You can find used sets for maybe half that.
I don't have the Titanic, but it's definitely on my wanted list.
There are cheaper large sets in the $100-$200 range that are very good and a lot of fun to build. Those $500+ sets are great, but unless you have plenty of space and time, I wouldn't buy much in that range.
I'm a collector and a bit of a Lego nut now, so it's something I really enjoy doing and it's worth it to me.