Google Photo offers unlimited photos. It's high resolution but it's not FULL resolution. The storage is free for pictures with a limit of 16 megapixels. 16MP is plenty! Most professional photographers 6-8 years ago, didn't even have cameras with higher MPs so it's fine.
Unless you want to print pictures 20 feet x 30 feet, megapixels are irrelevant. 16 MP is pretty much a 24 inch long side print at 250 dots per inch (super high resolution print).
Even if you want to backup RAWs or bigger high resolution files, you can use Amazon Cloud services. They charge about $0.0125 per GB of storage and it's scalable. So, if in one month or most months you only have 100 GB online backed up, then the bill will be $1
EDIT: Actually it's cheaper. FOr backup purposes you could use Amazon Glacier which is for archival purposes (long term backup and rarely accessed). The cost per GB is $0.004 ... therefore 100 GB is 40 cents, or 4$ per TB = 48$/year but again this is for long term "tornado happened to destroy all my backups" solution since it takes a couple hours to retrieve files.
You have to be really careful with Glacier pricing. They also charge to retrieve data past a certain point and it adds up fast (if you do only individual files at times that's fine, but if you're doing a full restore you can get in trouble). I remember crunching the numbers on it once and it was sobering to say the least. Probably worth coughing up if it's your only way to save everything, but I wouldn't set things up that way.
And FYI, the Canon 5D Mark II, the standard in professional photography in its heyday, was 21MP and released just over ten years ago. :P (Current iteration, the Mark IV, is 30MP.)
Sort of. From what i can see it wouldn't be that expensive. Bulk pricing of Glacier is 0.0025 per GB... Meaning if you wanted to restore 4 TB of data in one go, price would be around 20-30$. If you want expedite then it's 10-15x the price.
I talked to a friend of mine and he uses BackBlazed. It's 5$ per month and you have unlimited storage. You can retrieve data for free (its hot all the time) or you can also request in case of disaster, instead of having to download 50 TB of data which could take months... They would send you hard drives at a very premium cost. HOWEVER, if you return the drives, you get refunded the total price making it essentially free. However, they mirror your hard drives and is not additional data... if you delete anything from your harddrive it will be deleted from the cloud as well.
It would also be a set and forget approach.
Amazon Prime also offers unlimited storage of photos to prime members (I dont think they limit how high of resolution files can be as in Google).
Regarding the MPs... true, but I said most :D (just kidding) but I think you know what I meant. 16 MP is plenty for any hobbyist, enthusiast and really for any professional as well. You can print anything with 16 MPs really, of course you can't print at 30 ft wide at 300 dpis... but then again, dpis are dependant on viewing distance as well so it's a moot point.