What is the cheapest way to learn online?
Free via Khan Academy and the audit routes on edX and Coursera (all the material, no certificate), plus the free tiers of Codecademy and DataCamp. If it has to be a purchase, Udemy courses on sale (around $13 to $20, lifetime access) are the best price per skill.
Choose smartly and you can learn at top level for almost nothing. The royal road is auditing: on edX nearly every university course is free to follow (only the certificate and some graded work cost money), and Coursera matches that for much of its catalog. Khan Academy covers all the foundations completely free, forever.
The cheapest paid option is Udemy in the sale, and the sale is essentially always on: complete courses of dozens of hours for $13 to $20, yours for life. Domestika applies the same model to creative subjects. Never pay list price on these marketplaces; waiting for the promo is part of the system.
Subscriptions are only cheap with heavy use: $14 a month for Skillshare or DataCamp is a steal if you learn weekly and wasted money if you forget to cancel. And do not overlook the genuinely free corners: YouTube and free coding platforms cover an astonishing amount; paid courses mostly buy you structure and sequence.