DIY Catamarans
Many of the first catamarans were built in the garage by hobbyist builders. To build your own cat you need a good plan and plenty of time and commitment.
There are a few designers who sell building plans. The costs of these plans varies as well as the quality. The cheapest plans call for an investment of roughly $3000 plus the costs of the plan. The estimated time to build it is around 250-300 hours. So if you are able to invest 10 hours a week, you are looking at a good half a year.
Cruising Catamarans
For the cruiser, there are plenty of plans that require a minimum of 3000 hours of work. Because this is a bigger boat, perhaps 25-30 ft, or 7.5-8.5 meters, you probably need some machinery, such as a lift or a ceiling-mounted pulley and other special tools, and sometimes you need a few people to assist you, because the individual parts are too heavy for one person. 3000 hours means that it would take about 1.5 years of full-time work, or 6 years if you invest only 10 hours a week.
There are a few designers that will build the basics of the boat - maybe the first 2000-3000 hours, and then you can fit it out as desired, including sails and rigging, furniture, galley (i.e. kitchen) equipment, etc. That is an excellent option for people who want to have a hand in the construction but don't have much time (or space) on their hands.
Actual Examples
From the Time For a Catamaran Adventure homepage
This is the 28 foot (now 32 foot - Full Article) sailing catamaran that the builders launched in June 1999 after 28 months of serious part-time work. It is Richard Woods' Gypsy 28 design.
On the original website, they estimates that the value of the catamaran is $120.000 by the time it was finished. The actual expenses were $80,000 to make it off-shore ready. So by putting in his own labor of 3500 hours he enhanced the boat's value by roughly $40,000, which works out to about $11/hour. To make $11/hour if you are in the 30% tax bracket means you must make $16/hour gross. Of course, all these numbers are estimates, and there is no guarantee that the cat would actually fetch that kind of money if you'd tried to sell it. The moral of the story is that this is the kind of money (in 1999 dollars) that you will be working for when you build your own boat. Of course, the more important issue is that it comes with bragging rights.
The funny thing is that after they built the boat and used it for a number of years, they decided it was too small. So they cut it in half, inserted an extra 4 feet in the middle, and re-connected it. That's something you can do when you build your own boat.