It would be even easier if they were free! I always get maximum memory so mine was really expensive, but I didn't get the cellular version. My mini has that, but I've found I don't use it even though I get a little bit of data free with T-moblie.
As for replacing a laptop, some people do it and others don't. It really is going to vary based on what you do. I would certainly think anyone doing email, internet, movies, music, podcasts would be fine. Especially with a handy TB for serious typing!
Some do some serious creation stuff on them, but I don't see it as a replacement for me. For one thing, I have a 22.5 inch iMac and connected to that is a 24 inch monitor so I have quite a bit of space and I really like that. And there are things I do with a mouse that I don't see any good solution for with an iPad.
Heck, even webpages. I have a Logitech mouse with lots of buttons. Apple is notorius for making it difficult to maximize such devices so I have an app called Steermouse which works with this mouse to let me program buttons to do almost anything.
For exampse, I click the wheel to open a link in a new tab (nothing special there), but when I finish with that page, I have another button programmed to close the active tab. Another button enlarges the text every time I click it. With my eyes, I use that a LOT. Another button reduces text size the same way.
So, unless I have to type something, I'd doing everything with the mouse. My task chair happens co have a fairly flat armrest so my mouse just stays right there, convenient. My keyboard, when being used at all, will always be on my lap.
Most of my open apps (I have a number of them), are set up on screen so I can always click someplace and catch at least a corner of any one of them to bring up what I want quickly.
With an ipad, I can't gave that many things going on with easy reach all time. I'd often have to reach out to touch the screen to do some things as well instead of what I'm doing right now, leaning back in my nice chair with a big screen in front of me.
I probably should experiment doing stuff I normally only do on the computer, just to see what I can adjust too. If I do, I'll let you know how it goes.