The dealer will probably sell the backhoe, attachment hardware and bucket for around $7000. There is little markup and selling an add-on for much less than that figure is unlikely. There are however additional costs involved. Shipping of the backhoe, bucket and brackets is expensive. Installation will be probably 3 hours or so, at what, $60-80/hr? Tax on the whole thing will be hundreds. When it's all over you will receive enough change from your 80 $100 bills to take you and your spouse to dinner a couple times.
Normally around $6600 would be a fair price for the unit added as an option, but after the sale costs are a different story. I think mine came to a little over $7700 after Mississippi's 7% sales tax.
The new models are designated BH75A and will be reflected on the data plate as such. I don't think there are any 75's left out there. It would be a good idea to be sure though. From my own research, your $7300 (plus tax) seems to be a good price.