Here is my 2c on it.
Hosting make sense IF you take your time and plan your solution. Over time the value breaks even over self hosted. I know plenty of folks will disagree with me and that's their right but Having worked with both I will stand behind my experience on it. Now don't confuse the word value with cost. they are different things. Anyway about calculating your costs, it depends what you are trying to do. I'll circle back on that value in a min.
So in most base form you could look at just spinning up three virtual machines to recreate your current self hosted environment. You take care of your own licensing and apply it to you hosted systems. You create a small network in the hosted environment with your three VM's wall it all off so you can't reach it with out VPN, add an application gateway in front of your website so you do things like Geo Blocking. and you are good to go.
If you cost that out with straight up on demand it's going to cost you a fortune. Now if you are willing to commit yourself to at least 12 months, maybe even 36 months you can save anywhere from 25 to 60% cost with dedicated instance pricing. It really just means you are to commit to pay for X resources X time and they will give you a cost reduction so look at dedicated instance pricing.
Now to simplify your life and stop worrying about licensing most of these instances can also be purchased with licensing in place for the OS and SQL. This just removes a headache from the process.
Now where the value comes in on this? Well often you can clone your entire environment and have a cold spare ready to go in a second availability zone along with regular snapshot backups it can make for a very solid Business continuity case that would be difficult to match in a self hosted environment. add in the fact that you can scale up any of the instances on the fly if there is a sudden upswing in need and scale back down when done. it makes the process more cost effective than trying to roll out future proof environments in house. That is where the value comes in. I have complete clone of my ERP system on hot standby for about 20% the cost of maintaining identical cluster at two sites with way more bandwidth on tap.