These add little difference nutritionally, but make the world of difference in whether this diet seems bland or repetitive; with care they don't need to add a huge cost either, stored well they can be bought in bulk and the cost amortized.
If you want to buy cheap spices you can't go wrong buying whole dried spices (they keep their flavor much longer) from an indian grocery store. I've always found one near where I've lived (mostly east coast, but I did spend a summer in Albuquerque and I found a few there).
Not necessarily, though it depends on what you consider skyrocketing. I find that it only adds $2-$3 a day, depending on what your eating and how you buy it. I buy pork chops in bulk for around $2.49/lb. So that means I can add a half pound chop to my dinner for only $1.25. I also tend to stock up frozen chicken breasts, which I don't remember the cost of since a big back of those lasts me a long time. But I usually end up eating one pork item and one chicken item a day, alternating between lunch and dinner. I'd say in total my daily meals are $4-$6. Obviously if you're eating filet mignon as your meat, your costs are gonna shoot up. :)
Anyhow, this is an interesting setup. Rice and beans is a fairly standard South/Central American diet (for the same reason chosen here... high energy, full protein complex, and cheeaappp). This is totally a workable diet, but the moment you start wanting to eat fish/meat, your food expenses will skyrocket. Even if it's just Tilipia (yuck!). Which sucks =/