Britney Spears was doing well with her Vegas contract -- raking in nearly $30 mil for 2 years ... but we just found out, 50 shows into her run, it was so successful she got an even sweeter deal.
Sources connected to Britney tell TMZ ... the singer did indeed sign a contract for $310K per performance, and with 96 concerts over 2 years to total was $29,760,000.
But Planet Hollywood was blown away by ticket sales, which they said broke Vegas records. So Britney's manager went back to the bargaining table and hashed out an amendment, which now guarantees the singer around $475,000 a show ... Celine Dion, the top money earner in Vegas, makes $476K a show.
Planet Hollywood actually added shows for Britney ... she'll now do 140 shows during the 2-year residency.