we have had the same helper for almost 5 years. we have always given her: $500 in her christmast card, a few small gifts (we once gave her a dvd player from DCS ~$250) and some chocolates in her stocking. we usually give her bath products (it's become a joke in our house!) and she gets enough "luxury" to last the year (bath salts, scrubs, moisturisers, exfoliators etc)...
we've NEVER given a whole month's pay... we can't afford it.
we DO always invite her to have new year's eve dinner with us (we go all out on seafood: lobster, king prawns, alaskan king crab etc)