They work with you... at least my vendor from BG has worked with me regarding my budget.
But I also haven't seen any kind of PS discount listed. I see a price and a wire price... but the prices I've been getting quoted are lower than both.... just gotta get in touch with a vendor there.
BGD is not an advertising vendor on Pricescope. I know that many vendors (official and otherwise) offer a discount for wire transactions. Some of them also offer a PS discount (not sure if BGD is one of those or not) if you get there straight from PS.
Most of the reccomendations on PS are based on personal experience and word of mouth. Many people have had good experiences with BGD, and therefore they get recommended a lot. The same can be said for all the other vendors that you see recommended.
I was working with BGD and was told that without a doubt BGD does NOT give a discount on anything other than their signature stones (and I think blues as well). I was looking at virtual stones and was told those would NOT qualify for a PS discount.
All you can do is ask if the price is negotiable. Sometimes it may be and sometimes not. Can't hurt to ask. But as Gecko said, they are not PS vendors, so there is no reason to ask for a PS discount. Just ask how low they will go on the price of the stone you are interested in!