Seeing how expensive deli meat is it sure seems like roast beef sandwiches would be cheaper to make if you did your own roast and sliced it thin.
Are there any cons to this method? I imagine it would not last as long as the deli meat.
You've got a few things to consider --
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.
Roasting your own beef for sandwiches doesn't have to be time consuming, and you can easily get enough to make sandwichs for quite a while out of one roast. The only problem with slice your own is storage but your freezer will take care of that as long as you take care to make sure the packages you freeze in freeze as quickly as possible to prevent ice crystal formation.
The slicing problem is easy as long as you make sure you only cut your roast cold. You should be able to easily thin slice using only a knife, and while it will take longer than a deli slicer, you should be able to slice through a decent sized roast in only fifteen or so minutes.
It's all depend if you are going to eat all the roast beef you prepare. If you just need three/four sandwiches, then it's probably better to buy deli meat.