While bernd_k posted several alternatives for asking whether it's the last price, if you want to know what the last price is (and not whether it is one), try one of these:
- Was ist der endgültige Preis?
- Was kostet [das Objekt] am Ende / letztendlich / unterm Strich?
- Wie viel muss ich bezahlen?
There are, of course, a lot of alternatives if you're still negotiating. bernd_k's examples fall into this category. If you want to ask if the seller will go down, you can ask, for example:
- Kann man da noch [preislich] etwas machen?
In the example you found, the article is required, but I think you wouldn't actually say "Was ist der letzte Preis?" in German. At least I never heard that.