1. Online and desktop (installed on your own computer) have important differences in capabilities. https://ttlc.intuit.com/community/choosing-a-product/help/what-s-the-difference-between-turbotax-onl...
2. Intuit is the only seller of the online program, you are stuck with what they charge. Retailers can sell the disks for any price they like, even if they make little or no profit. Turbotax is not allowed to set mandatory prices.
3. Costco is a special case because they make almost all their annual operating profit on membership fees, so they can afford to sell products at or even below their own cost.
4. Be aware that some stores sell boxed copies of "Federal+e-filing" and "Federal+ one Free State + e-filing" because some states don't have income tax. Make sure you get the right box.
5. The boxed copy will charge you $25 extra to e-file a state return even if the state preparation module is included. However, Turbotax online charges $40 or $50 for state prep including e-filing.
6. The desktop version is almost always more cost efficient unless you qualify for free online filing. However with the desktop version, your files are not backed up anywhere and you are responsible for your own security and backups.
For more than that, I would have to give you a college level course in economics.
