@DML dude! There was a huge flame war debate over this recently which got very ugly but mostly over a misunderstanding. Poor guys can seduce and there's been subsequently posts how income has affected game, how to game when you're broke and a few other things scattered about.
There can be issues with different economic classes for an LTR but there's a lot of variables. If it's not a huge difference between your classes so that you're socially acceptable then there isn't an issue. If your world's apart there's issues on an LTR because of her social image, her social standing, the views of society, pressures from her friends but seduction as a whole is unaffected but you're usually an exclusive lover, fb, fwb, mLTR is the usual territory for this dynamic.
That said, if you're not a million miles away (classwise), you have good boyfriend potential, you're socially acceptable then if you're a leader, can manage relationships it's relationship maintenance 101 an LTR is viable.
If she cares a lot about social standing, keeping up with the Jones's, social ladder climbing then there's problems on the horizon. You'll have to reframe everything and have a few issues if you can't meet her expectations. If you can currently hang in her world and she's looking for someone to do the same if you can't keep up she'll move on. This is something you'll have to screen for and see what she's like.
If she commits to you, submits to you, you provide good dick, the more invested she is in you when you're not making the same money the more likely she'll keep around. Even if she wants to keep up with the Jones's then she'll look at your potential and the investment she's already made to you and be more loyal and accepting. How accepting depends on the girl, how much less class we're talking, societal pressures and how committed she is.
If you're convinced she's LTR material, you're happy with the screening you've made then it can be viable dude.