Ha, if you're willing to cough up a bit of cash you can probably get a Japanese one for $30ish, shipping and all. I bought mine for $10 on Yahoo Auctions Japan using a third party service, and spent quite a bit more on shipping and fees. Keep an eye open on there and you can probably find one.
The US version is much harder to find. Most of my GT collection has been amassed through eBay, but besides the GT4 Japanese Beta the US copy was my only purchase made with someone on a forum. I paid quite a bit too much at $50ish, but it's the only time in a few years I saw it for sale, and it was sealed too!
In case you or anyone is interested, here's a link to my collection as of February 1st. Both of the betas came after, and I have a second copy of GT4 BMW 1 Series Beta and the Gran Turismo 1 Test Drive disc on the way.
http://imgur.com/a/1hwfq