Okay, since you are talking 20" then it looks like T5 is out of the question as I think the smallest they come in is 24".
Here are some links for Power Compact fixtures that fall within your price range:
Coralife 1 X 96 watt 50/50 bulb, meaning half is 10,000k and half is actinic:
http://www.aquacave.com/20-1x96w-cor...-hood-634.html Price as linked is $109.95 plus shipping
Current 2 X 40 watt with lunar lights, 1 bulb is daylight the other is actinic and LED moonlights:
http://www.aquacave.com/20-2x40w-orb...t-usa-885.html price as linked is $129.95 plus shipping
Current 2 X 40 watt, 1 daylight one actinic no moonlights:
http://www.aquacave.com/20-2x40w-sat...t-usa-903.html price as linked $112.95 plus shipping
Or you could go with a 70 watt metal halide pendant system such as this Oceanlight:
http://www.aquacave.com/oceanlight-h...edic-1099.html price as linked $239.95 shipped
I just pulled these from a quick lookup on this one web site. You can use them as specifications to shop around some and try to find the best price. I am not affiliated with the linked store and am not trying to push their products in any way.
If you really want to keep Acropora then you may have to go the Metal Halide route.
HTH