Is it cost effective to make your own roast beef sandwich?
You've got a few things to consider -- 


cost of the ingredients
shrinkage
energy costs
time costs
wastage


So, in our decision making tree, we have to consider the real costs of each option.

Say for instance that whole roasts are on sale, so the cost of a roast is 1/2 the cost of buying the deli meat.  The roast is going to lose weight as it cooks ... it's just part of the cooking process.  It could be anywhere from 10 to 25%, depending on the cut of meat and how hot you cook it.

Obviously, there will be other spices and such, but we'll assume they're mostly negligible.  There will be the cost to power your oven ... and that'd be a function of where you live, if it's gas or electric (or solar), etc ... and if you make too much, and can't eat it before it spoils, then there's waste.

... but then we get to the most important part in my opinion -- time, and slicing.  Even if we assume our time costs nothing (and when you're throwing something in the oven and leave it for a few hours, that part's negligible).  Our real time comes in slicing it -- and I don't have a professional slicing machine, just a bunch of knives, so it's going to take me quite a long time, and I'm not going to get it as thin as the deli can.

And in my opinion, it's the shaving of the meat that matters; there's a big difference between large chunks of meat vs. thick slices vs. thinly shaved.

...

So, in my opinion -- is it cost effective?  maybe a little bit, if you assume your time is free.  Is it worth it?  Maybe if you want to spice it special, or make it lower salt, stay away from sulfides / sulfates, or do something else special ... but as you likely can't slice it the same, it's more likely to come out as an inferior product.

That's not to say that I won't ever do it, but I doubt I'd do it anytime soon, and it won't be because I think it's a cost savings.