You can build your own T5 HO setup pretty easily. Any kind of wood enclosure (hood) with a fan or two ( important! ) will do. You can keep it cheap by using low grade pine and painting it, and keeping to door / opening configuration simple.
You will need ballasts to run the bulbs, bulbs, reflectors ( important! ), and the hood w/ fans. I can give you more detailed instructions and tips if your interested. Ive built a few myself.
For a 6 bulb fixture ( max amount of T5 you can have ( 6 x 3" reflectors = the full 18" )) :
Ballasts; Fullham makes good cheap electronic ballasts. Stay away from magnetic ballasts, they are harsh on the bulbs causing premature ware. GE and Advance make good top of the line programed start ballasts. I buy from 1000 Bulbs, best pierces, service and selection.
F39T5 - Fluorescent Ballasts | 1000Bulbs.com You will need 2 of the cheaper ones ( $28 a piece ) or 3 of the expensive ones ( $40 a piece ). In your situation I would go with the Workhorse 5 ( $28 one ). Doesn't have all the bells and whistles but still high quality.
$56
Bulbs; On-line will be you best bet price wise. I recommend 3 actinic and 3 10k. Avoid Foster and smith as they do not know how to pack them. They tried to ship them to me 4 times. UV lighting Company has some really good stuff, right under Giesemann. Your looking at a bout $19-24 per bulb with 20 average. So we'll say $120.
Reflectors are worth there weight in gold to me. They help alot. Get the good polished parabolic ones. They cost about $23 a piece no matter the size. So instead of getting 6 34" get 3 56" and cut them in half. You will save about $69.
Total for all 3; $69.
The hood, there have already been posts in other threads about this so I wont go into it unless you are going to build one.
The fans are very important. If the bulbs get to hot they will self destruct. If they are taken care of they will last about 18 months. If let get hot only 2 or 3. There is no form of passive cooling that will work. They must be air cooled, they should never be hot to the touch, only warm. Cheap fans are noisy.
I'll say $30 for fans and $ 30 for the hood, but that can up depending on what you want.
So a 6 bulb top of the line DIY T5 HO we are looking at about $300. If you were to buy it you would pay $450 - $700.
A four bulb fixture would run you about $250 DIY.