No sense throwing good money after bad ;-)
You'll learn quick not to fall for any kind of negotiating tactic like that. If they lowball you on the first job with a sucker punch of coming back for more, what makes you think you'll get paid double for the second job to make up for it? They know you'll work for less, and now that they're a repeat customer they'll want even better pricing!
There's plenty of similar scams out there;
"give me a good price on this tree and I've got a tonne of other trees that need doing"
"give me a good price on this tree and my neighbour needs his tree done too, I'm sure I can help you get the job"
"I'm rich and I can introduce you to all my rich friends"
etc etc....
Just price each job for what it's worth, pay no mind to all the other fluff.
Shaun