Cum rights, on the other hand, means that your rights offerings haven't been exercised or expired yet, and you can still make a decision about what to do with them. Because you can often sell these rights to someone else, the shares with cum rights are generally worth more than those with ex-rights.
Estimating the theoretical ex-rights share price
Once the rights offering has wrapped up, the price of your stock is likely to change some -- at least, at first. If a lot of stocks were purchased, it's going to cause some dilution of the value of the shares, although they will likely rebound, especially if the rights offering was successful. A lot of buy-in at a discount drives the price down, but it also shows that investors are interested in the stock and have faith in the company.
To estimate the theoretical ex-rights price (TERP), simply add the current market value of all shares that existed before the rights offering to the total funds raised by the rights offering. Then divide by all the shares, including the new shares, on the day the rights offering is wrapped up. That looks like this:
TERP = (Existing Total Shares Value + Funds Raised By Rights Offering) / Total Shares on Expiration of Rights Offering
So, if the 10 million total existing shares were worth $20 million when the rights offering took place, and an additional $5 million were sold during the rights offering, which also happened to be 5 million shares, you now have 15 million shares worth a total of $25 million. This makes the new estimated stock price $1.67 per share, down from the original $2 per share before the rights offering.
TERP = ($20 million + $5 million) / (10 million shares + 5 million shares)
= $25 million / 15 million shares
= $1.67 / share