It's not really about high and low, it's more about valuation.  If you can peg your valuation at $20 per share based on a variety of factors related to your revenue, free cash flow, discounted growth, etc., but your stock is trading at $12/share, your company is undervalued and a share repurchase may be a good idea.  But if your stock is trading at or above $20/share, yet you can't make the case that your company is actually worth more than $20/share, it's foolish.