Hi, Samuel! I really cannot give you a price for a trip to a grocery store via taxi, as you would have to figure a round trip ride, plus the taxi waiting for you while you shop, plus tip. This would probably be quite a bit.
A few other options. One option is to order your groceries ahead of time on
www.gardengrocer.com. For a small fee (about $12, subject to change) they will deliver your groceries right to you at your resort. They do need advance notice, so check the website, or call them at 866-855-4350.
Another option is to do some light shopping at your resort. Milk, bread, snacks, and other items are available right in your Disney Resort Lobby.
For travel times around the resort, check out
www.ourlaughingplace.com. Go to the bottom of the page and click on Transportation Wizard. You can plug in any 2 locations and the Wizard will give you approximate travel times and how to get there. From Pop Century to Grand Floridian, the Wizard puts your travel time between 43 and 49 minutes depending on the route you choose.
Have fun planning your trip!