I have an "old time" water heater...?

with the big tank, I think it is 40 gallon. It sits about 2 feet off the ground. It has no jacket (may be insulated from the inside). The water heater is in my laundry room which forms the back wall of my carport (the plumbing wall is shared with my bathroom if that makes any sense), so technically outside and not climate controlled but not fully exposed to the elements either. The room has a door that I am willing to keep closed but usually do not. Oh yes, I live in Texas. Hot hot summers and mild winters.

The water heater is NEW. It was installed July 2012 when my old one blew out and leaked all over the carport. It has a vacation setting, then low to high to very hot. It is currently set just above low.

I am wanting to turn it up a bit, seeing how I can usually shower with water from the hot tap only and it runs out if I am using a lot of water such as a deep soaking type of bath (not very often, I do like to conserve water). But I kind of like my gas bill where it is.

I wonder how much more does it really cost to run a water heater on a higher setting. Does it double the bill? A few dollars more? Just in general, I am weighing my options. I need hotter water.

Of course if you have a better option instead of just cranking it up and using more gas, I would love to hear it.

Thanks!