Is this actually a good price?
Ignore the crossed-out number for a minute. Compare today's price with what the item usually costs, what it regularly drops to, recent lows, and what other stores charge.

Ask these five questions
You don't need to research for hours. These comparisons show whether today's price is strong or just looks impressive.
What does it usually cost, and what does it regularly drop to?
Look at the last few months, not just today's crossed-out number. A $79 offer isn't special if the product drops to $80 every few weeks.
How does it compare with recent lows?
Check both the recent low and the all-time low when that history is available. Today can still be a good buying price without setting a new record.
What are other stores charging?
Match the exact model, size, and condition, then include shipping in the total. A lower price on a different version isn't the same deal.
Is the list price believable?
MSRP is only a suggestion. If stores rarely charge it, the big percentage-off claim doesn't tell you much.
Is there a better option for the money?
Check a few similar products. A small step up in price may be worth it, while a cheaper option may do the same job.
Where this advice comes from
Google Shopping shows typical prices and recent lows. The FTC advises matching the exact item, comparing sellers, and including shipping and other fees in the total cost.