If you want to use internet trough data card (dongle) on roaming without extra cost, only few service provider is offering this facility. Here is the list of best option for you
1. TATA PHOTON PLUS- You need to buy TATA PHOTON PLUS dongle (DATA card) at the price of Rs 1199 that gives maximum speed of 3.1 MBPS. After that you recharge the data card, there are various options available. The best option is Rs 750 for 2GB (validity -1 Month). It will give you high speed internet in Mbps range with free roaming across India. Here is the more details.
2 . Reliance Netconnect Broadband+ – This also provides free roaming but, speed may not be in the range of MBPS. The Reliance Netconnect Broadband+ dongle cost is also Rs 1199. The best plan is for prepaid CDMA Network 1x speed is Rs 801 rechaege for unlimited data use (validity -1 month). More details are given here
To avoid huge amount of your balance deduction for data uses, it will always be safe to check the balance amount in your account before and after data use, so that you should get verified that money is not deducted as soon as you use internet. Never take any data plan which charge you on basis of use as 1ps/ 10 kb type of plan, it will deduct the huge amount for small amount of use of internet.
Before using internet be sure your plan has activated.