sh_config.lua

PS.Config.CalculateBuyPrice = function(ply, item)

    Everything half price for admins:
    if ply:IsAdmin() then return math.Round(item.Price * 0.5) end

    25% off for the 'donator' group
    if ply:IsUserGroup('donator') then return math.Round(item.Price * 0.75) end

    40% off for the 'superdonator' group
    if ply:IsUserGroup('superdonator') then return math.Round(item.Price * 0.75) end

    return item.Price
end