If the basic income was set somewhere near the Federal Poverty Level (FPL) for an individual, it would be ~ $1000 / month [1]. (This is just for a rough sense of scale. I don't actually think it's a good idea to specifically aim to match that number, but I suspect we shouldn't go much above it. That's a longer discussion, though.) That wouldn't completely cover Social Security for everyone—the average monthly retirement benefit is ~ $1300 [2]—but it would cover a large chunk. Further, the minimum eligibility level for Medicaid is 133 % of the FPL [3], so if we'd like to maintain our current standards, provision of a roughly FPL basic income won't obviate Medicaid for everyone. (Medicare's qualification criteria are primarily age based, so the number of eligible beneficiaries would be unchanged by the existence of the basic income.)
[1] http://aspe.hhs.gov/2015-poverty-guidelines
[2] https://www.ssa.gov/policy/docs/quickfact/stat_snapshot/
[3] http://www.medicaid.gov/medicaid-chip-program-information/by...