The difference between the price at which the arbitrageur purchases the share and the acquisition price is known as the "arbitrage spread" or "merger arbitrage spread". This represents the potential profit for the arbitrageur, and it's the driving force behind merger arbitrage strategies.

Here's the formula to calculate the arbitrage spread:

Arbitrage Spread = Acquisition Price - Market Price of Target Company's Shares

Where:

- Acquisition Price is the price per share offered by the acquirer in the merger agreement

- Market Price of Target Company's Shares is the current market price of the target company's shares

For example, let's say the acquirer offers to buy the target company's shares for $50 per share, and the current market price of the target company's shares is $45. The arbitrage spread would be:

Arbitrage Spread = $50 (Acquisition Price) - $45 (Market Price) = $5 per share

This means that the arbitrageur can potentially earn a profit of $5 per share by buying the target company's shares at the current market price and selling them at the acquisition price once the merger is completed.