If you have any commercial office furniture stores in your area check to see if they sell used office furniture. I found a used work station surface that was 8 feet across and 28 inches deep. It was supposed to be for an office cubicle. I got it for $50.00. Then I went online and found very sturdy H shaped tables legs for $22.00 each. I bought three of them. I installed the third one in the center parallel with the length of the desktop to keep it from sagging. Now I have plenty of work space.
you could try going to some local auctions, flea markets, sales. or you could try some used furniture stores, clearance stores.
as long as the legs and base are in good condition you can refinish the top.
$100 is not going to get you anything NEW that is built well enough for me to recommend. you might get a very cleaply made flakeboard or glass-top desk for that price range but expect a product only meant for one monitor. you will want a fairly wide (and deep if you plan on a semi-circle array) desk for eyefinity.
a local retail store was getting rid of old wood clothing tables a few years back. i grabbed one that was perfect desk height and about 3.5ft wide, 8ft long. a strip of formica banding and a new formica top and it looks and perfoms great. in total i spent under $30 for supplies.
if you aren't afraid to put some effort into fixing up furniture you can get some great deals!