http://www.taxisingapore.com/singapore-to-johor-bahru/
Have a look at these options, maybe?
You can consider a cross-border taxi, but bear in mind that it can only go to a designated stop in Singapore. You can use a regular local cab from there.
I must add that I haven't used these taxis in many a year! Not since the LCCs brought down the price of air travel between Singapore and Malaysia!