Rome2Rio suggests a bus + feet combination costing a total of 2€:

1. Take bus 361 from Schiphol to Lisse, Vreewijk

2. Walk to the gardens

Cheaper than this are walking and hitchhiking.

Rome2Rio and the 9292 journey planner both suggest a bus + feet combination, costing a total of 4.5EUR according to the 9292 planner (the rome2rio price seems out of date).

The journey would be as follows:

1. Take bus 361 from Schiphol to Direction Noordwijk ZH
2. Get off at Lisse, Vreewijk bus stop
3. Walk to the gardens

Cheaper than this are walking and hitchhiking.