Been researching myself and read everything from $1500 a month to skys the limit. Even the $1500 a month can be reduced if your workcamping and getting a site with electric, water, cable, internet free.
What I've done is write down my costs now, removed the ones I will no longer have (House payment, home insurance, second car insurance, etc.) and then added in the new ones I will have full timing (Higher gasoline budget if we move alot, Higher Trailer Insurance, Trailer Maintenence budget due to more use, etc).
I'm coming up with $2500-$3000 a month counting Health Insurance until we hit 65. Thats work camping for a site/electric/water. Anything else thrown in reduces my costs. Likely can be done cheaper but rather have more than enuogh than realizing I'm short after a few months.