Which welfare plans are you cutting? Social Security? Nope, because the average payout is 50% higher than $1000/m. Same with SSI/disability. You certainly couldn't replace any of the medical plans (Medicaid/Medicare) with anywhere near that amount.
The welfare plans that might be subject to being replaced only amount to a few percent of the federal budget. The bulk of the budget is those plans above, which would result in less benefits if replaced. You're not paying for this by finding lost coins in the couch cushions.
Yeah, as I said in my comment, the primary way to pay for it is to adjust marginal rates so that it's (mostly) a transfer from rich to poor. If you're rich, you'll get a $1k UBI and also pay $2k more in taxes (or $4k, or whatever). Probably for those at the median income it would be roughly neutral - but with the benefit that if you ever lose your job, or want to leave an abusive marriage, or want to try starting a business, you still have that $1k coming in.
Medicaid and Medicare being wholly replaced by a truly universal plan unrelated to UBI is probably a prerequisite for a UBI for a bunch of reasons. Social security would probably be unaffected, at least initially, and SSI/disability would likely either be reduced or gradually phased out, depending on the amounts.
I know the other various benefits aren't huge in the budget, but they add significant bureaucratic burden to both the state and the individual. Eliminating that would be a big benefit of UBI.
The welfare plans that might be subject to being replaced only amount to a few percent of the federal budget. The bulk of the budget is those plans above, which would result in less benefits if replaced. You're not paying for this by finding lost coins in the couch cushions.