Well I'm in a better position than you in only having one DS to worry about fares-wise, and so far DH has resisted any attempts to get him back over to the UK (he's Aussie, local family hence reason for this location) so it was only 1 adult and 1 child fare to cope with.
The thing that probably saved me though was joining two playgroups - and making myself talk to the other mums. Sounds like you've done this a bit anyway; and now you're looking for work so once you get a job, you'll have colleagues to talk to as well.
Mumsnet was also a lifeline - it provided a continuity for me in that I was still able to chat regularly to my MN buddies, so it didn't feel so "altered", iyswim.
I am a real "homebody" though, haven't had much of a social life for years, so not having much of one now doesn't really bother me - if you had a good social life before I can see how it would be harder for you to cope.
I think it is probably a blip - I was feeling pretty low before I went home that first time - and so far we have been lucky in that DH has been very accommodating of my homesick feelings (which do rise up after a few months) and let me go back before I get too down.
Some Aussies are a bit funny, I've found, in that they seem to be all friendly on the surface and say "oh you must come round" but it never materialises - the better friends I have here all have some UK connection, oddly enough! either 1st generation Aussie, or moved here when they were young, or have married someone from the UK! They're easier to get to know more, probably because they can talk to me about the UK and know I'll know what they're on about!
It will get better, I hope for you - has your DH any friends in the area? Mine didn't, he only had 2 he still kept in touch with at all and we've seen them once or twice each in the 2.6 years we've been here - so it was all down to me to find local people to talk to.
Also, I found the heat of the summer very oppressive and tend to get much grumpier in the heat - that may be another factor.