D'ain't no lines I know of.
This is really one of those things that can go from 0-100 in eight seconds, soze, sorta like tipping the valet.
You could invite them to your house-warming party (the most successful real-estate people network like a mutha and would consider this a great gift; of course on the downside you might have some new friends for life); you could get them something kind of expensive -- $100-$200 -- and exceptionally personal, like a day of spa treatments ('thanks for all of your hard work, now take some time off and get a massage' kind of thing); or something not quite so expensive -- $30-$50 -- and personal, like next year's planning calendar from Daytimer or Palm or whatever they happen to use.
They usually get nothing, I'm guessing, beyond a thank-you note and less occasionally something like an appreciation gifty, which I would consider to be a plate of cookies or a basket of fruit or some bath salts; and once in a great while they receive something they would take to be spectacular, which is subject to your imagination and may or may not be expensive.
To extend the valet analogy, when I was one I often got nothing or $1 (hey, it was the mid-'70's); I once in awhile got $5; once I got a twenty, and once I got a fitty
with the lady's name and phone number written on it.
So, as you already know, it's really up to you whatchoo wanna do.