User blog comment:Spike144/Bloons Wiki News - February 23, 2013/@comment-3396321-20130223145340

In Defensive Mode, you start with $50 income. Replacing the ability to send Bloons, you can buy income.

$2 Income costs $50.

$15 Income costs $300

$70 Income costs $1,000

$700 Income costs $7,500.

Since players can not send Bloons, it takes a while to get a good defense up. But luckily, you don't have to worry about Rainbow Regen Rush/Ice Stallers/Camo Leads. :)