Cost of 3G
When is it cheaper to buy a 3G iPad than to upgrade your current iPhone to tethering?
Below is a calculation of the number of months of service required before the iPad 3G premium plus service is a savings over just upgrading the existing plan to tethering.
Using these base values from AT&T service:
iPad 3G Premium (one time cost) of $130
iPad 250MB Plan of $15/month
iPad 2GB Plan of $25/month
iPhone 250MB Plan of $15/month
iPhone 2GB Plan of $25/month
iPhone Unlimited Plan of $30/month
iPhone Tethering Plan of $45/month
To calculate the number of months of service where owning a 3G iPad with service is an overall savings over upgrading to tethering we get the equation:
Premium for iPad with 3G plus cost of iPad plan times x months = x months times (Cost of iPhone Tethering minus Cost of current Plan)
*Taxes and possible discounts are not taken into account
iPad 3G 250MB Plan v. iPhone 250MB Plan
130 + 15x = (45 - 15)x
130 = 15x
x = 8.67, after 9 months you save $15 per month
iPad 3G 250MB Plan v. iPhone 2GB Plan
130 + 15x = (45 - 25)x
x = 26, after 26 months you save $5 per month
iPad 3G 250MB Plan v. iPhone Unlimited Plan
130 + 15x = (45 - 30)x
130 + 15x = 15x
*x drops out, poor x, same cost per month for either service, but $130 premium for 3G iPad
iPad 3G 2GB Plan v. iPhone 250MB Plan
130 + 25x = (45 - 15)x
x = 26, after 26 months you save $5 per month
iPad 3G 2GB Plan v. iPhone 2GB Plan
130 + 25x = (45 - 25x)
130 = -5x
x = -26, you lose $5 per month from the start plus your $130 investment
iPad 3G 2GB Plan v. iPhone Unlimited Plan
130 + 25x = (45 - 30)x
130 = -10x
x = -13, you lose $10 per month from the start plus your $130 investment
x represents the number of months of service it takes for the 3G iPad to be a savings per month over upgrading to tethering. When x is a negative number you are losing the initial premium plus you are losing money each month by purchasing a 3G iPad and having a data service over tethering.
The ideal situation for purchasing a 3G iPad is when you have a 250MB plan on your iPhone and only intend to get a 250MB plan on your 3G iPad. In this case, after 9 months of service you are saving money each month on the iPad 3G plan over just buying wifi and tethering.
These calculations do not take into account the joy of owning a 3G, the GPS chip usage (you do have an iPhone already with one), or the hassle of having to tether each time you want to connect your iPad to the 3G network.