I got a chrome-plated, open-wired, shelving unit at Costco for about $80. It's very sturdy, comes with 6 shelves that measure 18" X 48", and the four supporting poles are each made up of two 3' poles so you can configure it as one 6' tall unit with 6 shelves, or two 3' tall units. It also comes with four large wheels so it can be moved around if you choose to put these on the legs.

It's been in my GH for almost 3 years and shows no sign of rusting or corrosion (I do typically keep my plants in large trays so I can bottom-water, and can avoid getting gunk coming out of the pots and crudding up everything).

I took a look at the Costco web site and it looks like it costs about $100 now. I've heard you can also find similar shelving units at Wal-Mart, etc. I could have built something for less (if I didn't count the hours it would have taken me), but I liked the fact that I could unpack and assemble this in a few minutes, and it's more functional and sturdier than anything I could construct, and the shelf heights are adjustable.

Here's is a link so you can see what I'm talking about (oh, and I don't have any connection with Costco)

Here is a link that might be useful: Shelves

Here's two options at walmart for less money, if you are looking for that:

http://www.walmart.com/catalog/product.do?product_id=8282891

http://www.walmart.com/catalog/product.do?product_id=8282890

They won't corrode, since they are plastic...but you might want to be wary of UV causing a problem.

There's a good post from Laserfan about 2/3 of the way down this thread, and he lists the materials he used to make his shelving:
10x12 HFGH Lessons Learned

I thought his shelving was very nice too, and the way he anchored it to the back wall and foundation added a lot of strength to his greenhouse...a very smart design!
Sheri

Thank you Sheri! I can say after 2 years now that the shelves are still rock-solid, rust-free, and one of the best features of our GH still! To flowermomof2 follow the link in Sheri's post; in THAT post there's a link to Photobucket where you want to find my Details sub-picture album. BTW maybe this is semantics but these are "shelves" not "benches". The cattle panel I used would not be very comfy to sit on! ;)

@sheri I don't come back here much anymore but as of today my ceiling panels are worse than ever--many dozens of holes have developed, looking like I've used one of those paper hole-punches on the roof! I never did install the new panels I bought. At this point I'm going to wait til next year I think to replace all the ceiling panels.

I believe they've become brittle and the newest round of holes might have been caused by tiny hailstones in an ice storm this winter. Bye...

back@laserfan,
good to hear from you, but sorry about the panel report. Not surprised; mine have continued to yellow; so far no holes (no hail) but I have a crack by one of the screws, which makes me worry screws could eventually pull through in bad winds. Heading into our windiest time of year (40mph expected today) so we're gonna pick a calm day, pull off the roof shadecloth, and do a thorough inspection...then decide next steps. (Insert bad words here.) Best of luck to you!
Sheri