If you use the Sightstone in most efficient manner, you will place two wards every third minute. A regular green ward is worth 75 gold.

2x75/3 = 50.

If you would ward as efficiently with regular wards, that means you are spending 50 gold per minute on wards. 

700/50 = 14.

If you compare that to the one time cost of 700 gold for a Sightstone, you will have made up the initial losses after 14 minutes.

(700-280)/50 = 8.4 (round up to 9)

If you sell the Sightstone, you will get 280 gold back. That means that you will have made up the initial losses after 9 minutes.