Where and how you can gather petitions is regulated for starters. Usually you can't do it on provate property without the permission of the owner.
In Nevada, which is very small state, you need around 83,000 signatures to get something on the ballot. The printing costs for the petitions, paying for public notaries, and the legal costs for writing the petition alone will set you back at least 50K to 250K. (Proportionally more for larger states.) The big issue is being able to get eighty thousand signatures (10% of the voting turnout in the last election is how the number is set) in about six months time. If you assume an average gatherer can get 50 petitions a day, and only about 80 percent of those will actually be registered voters or are not duplicates leaving you with 40 valid signatures, you need approximately 2000 man days of labor to get an issue on the ballot.
In six months (180 days) this means it is theoretically possible to do it with just 10-12 people provided they have no other jobs, family concerns, needs for breaks or time off, don't have to account for travel time, and they don't face inefficiencies from overlapping territory or coverage. This is not a possibility for a volunteer force. With volunteer work, at best you can assume people will work two days a week, and then not for a whole day (This is hard gruelling work, especially in Nevada because of the desert heat.) At most you get a 1 man day worth of labor per week from a volunteer, meaning you would need an average of about 100 volunteers over a six month period of time. Add in a manager for every dozen or so to insure everyone has their own territory and to arrange transportation and you're at 110 people. Add another five for legal staff and coordination of the statewide effort and you have 110-120 people.
The difficulty of the work also complicates things. I have never been yelled at spat at, called a communist, called a traitor, etc. more in my life when I worked on a raise the minimum wage petition. Add in a hundred degree desert heat, being chased off by security or property owners, hassled by police (A legalize marijuana petition. They wiretapped our offices and set up stings against our volunteers. This needless to say discouraged alot of people from helping), and you will find you have a turnover rate of well over 75% within the first two weeks. This increases the number of volunteers you need over the course of the signature run by a factor of ten easily. I'm guessing you would need 1000-1500 volunteers because of turnover rates over the course of those six months to gather enough signatures. To get that many volunteers you would need an extremely solid grass roots network and a proficient media campaign, meaning in all likelyhood that you will need paid professionals yet again.
Given a population of about 2 million, you would think it is possible, if highly improbable, to get that many volunteers, but I haven't seen it done. The numbers explain quite simply why every there has never been a successful petition in Nevada without paid workers. Every single one that tried to get on the ballot through only volunteer work has failed. In California it is the same. No petition can be done without paid professionals. The size of the task at hand, the huge numbers of signatures that must be gathered is just too much.
Money money money, it just can't be done without wads of cash. To give you an idea of the amount of money involved, in California alone in the 2005 initiative campaigns 200-300 million dollars were spent.
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http://www.sfgate.com/cgi-bin/article.cgi?file=/c/a/2005/11/09/MANDR.TMP>
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http://www.ppic.org/content/pubs/rb/RB_1198EGRB.pdf>
The power of money, if anything, is more blatant in initiatives than in normal politics. No money, no initiative. It is that simple.