Antonym of "low-ball" as price
According to Wiktionary, the answer is highball.
"Lowball" means:
To give (a customer) a deceptively low price or cost estimate that one has no intention of honoring or to prepare a cost estimate deliberately and misleadingly low.
To make an offer well below an item's true value, often to take advantage of the seller's desperation or desire to sell the item quickly.
Thus, the direct opposite would have to be "Highball" :
To make an estimate which tends toward exaggeration.