To find out how much you can sell a bond for go to the website I've linked below.
For the example of a 30 year bond paying 2.5% interest you will need to put into this calculator a maturity date of April 2042, a coupon rate of 2.5%, and a price of 100. On the top bar you will then see that your 2.5% bond amazingly "yields" an honest 2.5% annual return on your investment.
Then slide the "yield" bar to 8% (to represent the coupon rate of an 8% yielding 30 year bond) and watch your initial principal value of 100 (dollars returned on the dollar initially paid) vanish in "price". "Price" is the value (as a percentage of what you initially paid) that someone will actually be willing to offer you for your 30 year bond if you decide to sell it and go elsewhere with your money rather than hold it to maturity.
Here is the link:http://www.smartmoney.com/calculator/bonds/bonds-calculator--bonds--bond-funds-1309988621833/
You will be able to recover only about $37 for every $100 you initially paid for a 2.5% coupon bond in an 8% yield environment. And if interest skyrockets to 15% (as it did in the late 70's to early 80's as I recall), you will only be able to sell your 2.5% interest (coupon) bond for a bit less than $18 for every $100 that you initially paid for it. That is real wealth destruction.