Wow. Brilliant. Wish I had thought of looking for the lowest price per mile. Can you say "stating the obvious"? Okay, I am giving you a hard time. I'm just joking. I can't help but chuckle. So often a new FTer will ask "what's the best airline program?" not realizing that if there were a best airline program the others would dry up (or at least shrivel).
Usually not directly from an airline. And think twice before considering buying miles. Usually the price is not worth it. It can be a good option if you want to top off an account that is just short of enough miles for an expensive itin. However, you are trading cash that can be used for a confirmed seat on the flight of your choice for a lottery. With capacity controls, blackout dates, etc you are limiting your options using miles that you wouldn't experience if you just bought a ticket with $. Avoid buying (lots of) miles from airlines.
Some (I won't say "most" because I haven't done the leg work) airlines limit the number of miles one can buy within a calendar year.... 60K...50K... amounts like that. So buying 100K+ miles is usually not an option. Even with 100% bonus promos you are spending a lot of money for the possibility of getting the itin you want.
Do you mean paying your federal income tax with a miles-earning credit card?
Again, can't see why you would want to buy bulk miles unless you are a business that want's to offer miles to your customers as a perk.
When you find out let us know. There have been some reports on FT about how much Avis, Hilton, TIB, etc pay per mile but I have yet to see any definitive numbers.
When you use a partner of an airline in essence you are buying miles. So keeping an eye on many forums at FT will be needed as part of your scheme. MilesBuzz is a good forum but many times a really great mileage earning opportunity is posted in the forum for that airline or the forum for that partner. Example: until the end of May Budget was offering a 5K bonus of Delta SkyMiles for a two-day car rental. The thread for that promo could have been started in the Delta forum, Budget forum, or MilesBuzz forum. Since cross-posting is frowned upon at FT duplicate threads on one promo are likely to be merged into one thread. From my experience the older(est) thread will be the hub for the other threads. So keeping an eye on many forums at FT is the best advice I can give you.

Here are a few tips that get repeated often at FlyerTalk:

- Consider the Starwood American Express card. It is often touted as one of the better cards for maximizing points/miles. If you are loyal to a particular chain of hotels and don't use Starwood hotels much then you might find another card to be more lucrative for yourself.

- Several credit cards allow one to earn card-specific-points that can be redeemed for $ amounts towards air travel on any airline. The real advantage to these cards is that the ticket is actually being purchased so you will earn miles on the airline flown even though your ticket was purchased (in whole or in part) with points from that credit card.

- Check out the thread in MilesBuzz titled Q2 2010 Car Rental Bonus Promos!. I maintain post #1 of that thread with various miles bonuses gathered by myself and many others who rent cars frequently. There are instances when the miles earned are so lucrative for the $ spent that you might even rent a car even if you don't need one.

- Keep an eye out for and register for EVERY promotion you can find. Even if you don't think you will actually end up using the promotion ... ya never know. Think of this time as a part-time job. I glean so much knowledge from FT that saves me money and earns me more miles that I consider my time perusing FT to be my part-time job.

- Consider attending the October FT DO in Chicago. You can learn a lot from the birds-of-a-feather that will be meeting there to trade tips, tricks, and deals. There is a link to the thread for that DO in my signature.