What is the best/cheapest way to get from Amsterdam airport to Paris when you don't have exact dates on your arrival? (flying standby)
I don't know the stand-by rules for your airline, so I don't know the approx. range of dates you might be talking about, but you could consider buying a couple of days worth of (cheap) advance-purchase rail tickets from Thalys.com; they could be 35€ for the lowest fares (2nd cl). A walk-up fare will be closer to 129€ or 205€, depending on whether 2nd class is sold out or not. Blowing 35€ might be easier to live with ;-) Heck, even blowing 70€ would be easier...you can purchase tickets online 90 days in advance of your travel date, and that's when I would try to buy them, too!
You can investigate buses on http://www.eurolines.com. Looking at tickets for tomorrow (so, no advance purchase discount), they cost €46 (if you decline the insurance) and take 7:15. The bus station is reached by Metro from the main train station, and that in turn is a short train ride from the airport.