in Schuette v. Coalition to Defend Affirmative Action. The opinion is available here.

Here are the relevant passages….

Cite as: 572 U. S. ___ (2014)   19
SOTOMAYOR, J., dissenting

And the costs of qualifying an amendment are significant. For example, “[t]he vast majority of petition efforts . . . require initiative sponsors to hire paid petition circulators, at significant expense.” Segura Brief 10; see also T. Donovan, C. Mooney, & D. Smith, State and Local Politics: Institutions and Reform 96 (2012) (hereinafter Donovan) (“In many states, it is difficult to place a measure on the ballot unless professional petition firms are paid to collect some or all the signatures required for qualification”);

 

20 SCHUETTE v. BAMN
SOTOMAYOR, J., dissenting

In 2008, for instance, over $800 million was spent nationally on state-level initiative and referendum campaigns, nearly $300 million more than was spent in the 2006 cycle. Donovan 98. “In several states, more money [is] spent on ballot initiative campaigns than for all other races for political office combined.” Ibid.