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For an accurate idea of costs, you should contact travel agencies in Osh. Note that you will have to pay for the driver's return to Osh, and so the costs may prove higher than you expect. In Osh in August 2009 I was quoted $300-400 for a jeep to Khorog, the difference being in the quality of the vehicle. I suggest you pay for quality as the region is not a great place to go looking for spare parts and mechanical support. But in case of breakdowns or other delays, you also need to carry sufficient food, water and warm clothing to support you for a few days.

Driving non-stop apart from rest breaks and overnight stops, I suggest it will take two full days from Osh to Khorog, and about 24 hours from Khorog to Dushanbe to allow for delays due to road and traffic conditions. But this is a speedy trip, pure commuting, and a wasted opportunity. The travel agents in Osh will have some ideas on when and where to stop for sightseeing and local tours, and this will add to the time and expense of your itinerary.

To travel in the Wakkhan region of Afghanistan means diverting from the Pamir to go to Ishkashim/Eshkashim. This will add some days to your journey. You will need to obtain your visas for Afghanistan before you enter Tajikistan, unless you want to try your luck with the consul at Khorog and then backtrack. Note, however, that some recent reports on TT have indicated that the consul there is no longer issuing tourist visas. It's likely you will have to walk across the bridge to Afghanistan and pick up transport on the other side. You can pre-arrange this, but you'll need to research other posts (or elsewhere) to discover how. The TT contributor Claireredhair posted a detailed account of such a journey in (I think) early 2012 (or maybe late 2011) but I that post is not yet back online following the overhaul of TT.

You also need a special permit to travel in the Wakkhan region of AF, and so you'll need a local contact to arrange that as well as transport and food. It's a very remote with little in the way of services, and is principally of interest to trekkers. I suggest that 'one day or two' in the Wakkhan corridor will prove to be an extremely costly proposition, and maybe not worthwhile for that reason.

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The prices for living in Central Asia has gone up tremendously. Fuel prices have gone up tremendously (at the moment about 1.5USD/liter in Tajikistan). Therefore transportation has gone up a lot as well.
At the moment in Tajikistan one kilometer is 1 USD and on top of that 25 USD per day for the driver if you hire a driver in Osh.
Surely the quality also has gone up. These days you do travel in a Toyota Prado if you want.

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You should hire a jeep for each leg rather than whole way....so much cheaper. Osh to murgab, murgab to Khorog, Khorog to Dushanbe. Jeeps are plentiful and easy to negotiate. When u get to murgab ask for gulnara, she'll help u and offer her place to stay. Tell her Edik's American friend sent you.