Crepes and bistro in Paris
Very far from a croque-monsieur but there's an inexpensive option in the 8e that I like to rely on when I happen to be in the neighborhood: Le Baron is a small Lebanese restaurant on rue Washington where a plate of chicken chawarma is better than a good croque-monsieur.
For croque-monsieur proper, I'd head for the large cafés, pubs and brasseries around the place de l'Etoile, on all the radiating avenues (Grande-Armée, Wagram, Carnot, etc.) and in the Ternes area, or for the big posh cafés on Avenue Matignon (the Berkeley has to be tried, even once in your life), but preferrably not on the Champs. These big cafés tend to be old, reliable institutions and you might have good surprises. I like L'Orée des Champs, corner Ternes/Mac-Mahon.
The 8th is rather large. If you're near it, the creperie on rue de Washington is decent, as is the one on rue du Commandant RIvière. But good creperies are in Montparnasse (and Censier, Le Pot-O-Lait, but it's closed that time of the year). As for reasonably priced, there's none of it in the 8th. I take it that by bistro you mean a café where you can have a simple fare (as opposed to a not too expensive restaurant). Then my advice (including for croque-monsieurs) is to look at your neigborhood café and pick one who looks good. There rarely is such a thing as a good café because cooks change very often, so it's hard to recommend one.
I like the croquemonsieurs at Dalloyau, by the way, but they're atypical -- without crust and without cheese on top.