I would say at $100 you are getting a decent price
. Checking around they seem to be selling for more than twice that. I doubt any other alternative is going to be much cheaper and probably much less durable.
As far as batteries go with your current
setup if you ran a toaster you could run it for about 20 minutes before your battery
bank is depleted to about half capacity. That is always the best low end target to discharge deep cycle batteries so you get the longest lifetime. When you take them lower the lifespan decreases faster. The down side with inverters is they take 10 times the DC amps to make AC amps. You also need more starting amps than the amps to run anything.
For TV's, the plasma TV's are the killer consumers of amps. We run a small 15 inch LCD (has speakers too) with a 12 volt CD/DVD player and it runs fine on a 300 watt inverter. Do the math on the power before you get too far into this. Inverters are great but when you start running AC appliances
the battery bank account can be wiped really easy.
If you are going to add 2 more batteries it is best to start with 4 new ones and not just add two more. Battery Outlet in Chesapeake or Yorktown are probably the best place to buy them too. They have a web site but they carry more batteries in the store than they sell over the Internet
You might consider switching to 6 volt golf cart batteries divided into two banks. That could triple your capacity. With 4 batteries having two banks minimizes the problems with on bad cell taking out the whole bank. Also it's a good time to evaluate your charging
system. Batteries have got a lot more expensive with the price of lead going sky high if you have not priced them lately. Protecting your battery investment is always a good thing.
With more batteries you'll need some switches. A 1/2/both/off switch would be nice to make two banks. The supply lines to a 1000 watt inverter is going to carry a lot of DC amps. Volts X Amps = Watts! Size the DC supply and ground wires properly based on the distance. Once you start running these extra large cables
the cost of the installation
is going to increase. Don't wimp out on the installation
quality. This is not something to screw up or you will be on fire. A 1000 watt AC load is about 100 DC amps!
section of the forum has an almost unlimited amount of battery / electrical
information you can go through. If you are not sure about your installation use this knowledge base and post more questions.