Airtel broadband (not 4G or 3G) has secretly launched addon packs available via it’s customer care.
These addon packs are available for customer not satisfied with their plan and demand additional data.

To these data hungry customers care suggest these add-on packs which are:
20GB @ Rs.100
50GB @ Rs.200
75GB @ Rs.300
100GB @ Rs.400

Though if you visit this smartbyte page the option are different and costly so better to call 121 and opt for upgrade. These addon packs are recurring which is different than smartbyte packs which are one time. So you need to add or remove these packs thats why are cheaper. However both addon and smartbyte packs are not pro-rate which means you would be charged for whole month regardless of data of activation in the billing cycle.