We can't really afford to go either. (and I totally hear you on the guilt thing)
I've been in the UK since Nov 2010 and I've only been back to the US twice - one was for a company trip (they paid for the ticket) and the other one was when my US family pooled their money together for a plane ticket. Both times it was just myself who went as that's all we could afford. DH had to stay home.
Saving up for future visas is definitely a big contributor, but it seems airfare has just gone up since I first started my LDR. When we first started dating, we could afford to see each other about 4 times a year (me to UK twice, him to US twice). Now, we would be lucky if we could afford one ticket to the US. Plus, it doesn't help that most people have the most free time during the summer and ticket prices just skyrocket around then.
I've found that the best way to get cheap tickets is to try and travel during really dead times... not during holidays, not during the spring/summer, choosing odd days to travel on, odd times to travel, utilizing discount websites, etc.
My family keeps asking when we'll come back to the US to visit them, and it's sad but true that we just can't afford it for the foreseeable future.