There is no minimum salary requirement for sponsoring your parents on Visit visa but it is good if you or your parent have healthy Bank statement Balance.
supporting Documents you need provide are :-
1- Sponsorship Declaration Form (which can be found on UKBA site)
https://www.gov.uk/government/uploads/s ... p-form.pdf
2- 3 Months Pay Slips
3- 3 Months Bank Statement (showing salary coming into Account)
4- Rental Agreement
5- Property Inspection Report (no 4 and 5 not required if you Booked any Hotel for their stay in UK)
6- and attested photocopies of your Passport and Resident permit.
I always send these documents whenever sponsoring to my mother from Pakistan and she always get visa.
Seniors members correct me if i am wrong.