I have a forum with roughly 3,500 members (average) and it gets between 500,000 and 600,000 pageviews a month.

Yet even with cache enabled the forum alone, despite seeing very few daily postings, takes roughly 30 - 40GB's of bandwidth! This is utterly ridiculous and I simply can't believe how poor it is.

I can't upgrade because v6.1 isn't out of beta yet and despite that it's 'Category Only' interface is likely to be just as poor and I don't relish having to re-design it myself yet another time.

Any suggestions on why this is happening or how I can really kill some of the bandwidth?

There are two possiblitys to reduce bandwidth:

1) UBB File Compression
http://www.ubbdev.com/ubb/ultimatebb.php?ubb=get_topic&f=30&t=000049

This "hack" works also with UBB 6.0x but require that Compress::Zlib is installed on your server. Read the entire thread at UBBDev for more info

2) UBB Accelerator
included in the upcoming UBB 6.1.0 Beta Release 2 and higher

This addon use the gzip output function of PHP 4.0.5 and higher to compress the output of the pages.

I'm fond of the file compression idea, although most hosts don't lift an eyelid when you ask them to install the required modules, so it's no good.

As for v6.1, there's no way I'm going near an Infopop Beta for the new series unless it's 99.9% final, too many bugs and dangerous problems.

You seem to misunderstand the purpose of the cache... the cache is server side, meaning that the UBB scripts themselves use it, not the clients. The UBB just sends the premade cache page to the user rather than generating a copy from scratch. This will use the same ammount of bandwidth as doing it any other way.

Some hints... turn off the iframe on the reply page, use 15 posts per page for the threads and 25 topics per page in the forums. Try and eliminate any graphical header or footer, if you have any...

I've already tried what you suggest, had it like that since Day1, in fact I was surprised you couldn't have less posts per page shown? Why 15 minimum?

Just a nice round number.. I've modified it to one post/page for a special application. It's removes your support options tho since you'd modify the files.

You can try the compression modification and see if your host already has the necessary modules installed, quite a few do and you know immediately once you try it. It only requires 3-4 lines added to ultimatebb.cgi and uploading the included perl modules (be sure to do this in ascii).

That compression system is for v6.1, not v6.0x.

UBB file compression works also with UBB 6.0x

Please read the entire thread at UBBDev

Looks like my only problem is to install the module itself, which my server won't do and it can't run from the cgi-bin rout.

In other words, just have to hope Infopop release v6.1 soon or my server will boot me for bandwidth usage.

No sir, if the other perl modules are already compiled in, it's a matter of uploading the cgi_buffer.pm as instructed and adding those 3-4 lines in. Quite a few servers already support the required modules, it would take less than 5 minutes to find out.