The 15" alloy wheels from the Ertiga costs Rs4990/wheel.
Check the price for the alloys at different place, your dealer is cheating you.If you go for 15"alloys you will need to change your tires, if you go for 14" alloys you can go with your existing tires.
If you decide to go with 15" alloys then try to get a higher price for your existing rims and tires,since they are almost new.
You can try to get the alloys from an MGP dealer as well.Check out this website: www.mgp.co.in
In comparison to Lenso and other brands they Maruti wheels do not look great, but they are of higher quality.