Every major loyalty program sells their points for cash, normally at a fixed rate through the industry-sponsored site Points.com.
For example, you can buy up to 60,000 Delta SkyMiles per calendar year for 3.76 cents each, up to 75,000 United MileagePlus miles for 3.76 cents each, up to 150,000 American AAdvantage miles for 3.19 cents each, and up to 60,000 Alaska Mileage Plan miles for 2.96 cents each.
Hotel programs likewise sell their points currencies for cash, with IHG Rewards Club selling up to 60,000 points for 1.15 cents each, Hilton HHonors selling 80,000 points for one cent each, Marriott Rewards selling up to 50,000 points for 1.25 cents each, Starwood Preferred Guest selling up to 30,000 points for 3.5 cents each, and Hyatt Gold Passport selling up to 55,000 points for 2.4 cents each.
Purchased points are too expensive for me
I don't personally buy miles or points because it's a more expensive way of acquiring miles and points than the other methods I have available.
United MileagePlus miles and Hyatt Gold Passport points cost just 1 cent each when purchased with Ultimate Rewards points transferred from a Chase Ink Plus account.
I happen to have a Citi AAdvantage Platinum Select MasterCard, so if I ever needed to stock up on AAdvantage miles, I can do so for 2.105 cents each — the cash back I'd earn manufacturing the same unbonused spend on my Barclaycard Arrival+ MasterCard.
And of course I earn 6 HHonors points per dollar spent with my American Express Hilton HHonors Surpass card at grocery stores, so even compared to an "optimal" redemption rate of 2 cents per US Bank Flexpoint, I'm already buying HHonors points at a mere 0.67 cents each, 33% less than the 1 cent per point Hilton wants to charge.
Purchased points may make sense for you
As the examples above make clear, the decision whether to purchase miles and points or manufacture them rightly depends upon your next best alternative: your opportunity cost.
If you're currently manufacturing the bulk of your otherwise-unbonused spend on a 5% cash back card like the Wells Fargo Rewards Visa during the introductory promotional period, then manufacturing spend on a one-mile-per-dollar card costs not 2.105 cents per mile, but 5 cents per mile, 57% more than, for example, American is willing to sell them!
Likewise, if you have $100,000 on deposit with Bank of America, you might be earning 2.625% cash back with a BankAmericard Travel Rewards card. That may make purchasing Hyatt Gold Passport points at 2.4 cents each worthwhile, compared to manufacturing spend on a Chase Hyatt credit card.
Purchase small numbers of points for high-value, upcoming redemptions
While you usually see affiliate bloggers advocate buying large numbers of points speculatively when loyalty programs offer the highest bonuses on purchased points (bringing down the cost per point), I have exactly the opposite view.
If you find yourself with an upcoming, high-value redemption, and don't have the time to manufacture the required points, then go ahead and buy them. Paying "too much" per point, if it drastically brings down your total out-of-pocket cost, makes perfect sense: the goal isn't to pay as little as possible per point, it's to spend as little money as possible on the trips you actually want to take!
But the money you spend speculatively buying miles for redemptions you don't actually have planned could almost invariably be better spent building a credit card and manufacturing spend strategy that generates the trips you want to take at far lower out-of-pocket expense.