Sounds like you got a really helpful dealer! As you say, it was a learning curve for him and he can now offer a sim-cutting-down service. Telcel company stores should (in theory!) have microsims in stock.
The rates/deals for the iPad are the same prepaid Amigo rates available for any other data connection (what was new in Sept 2010 was the launch of the microsim). Note the double-data offer mentioned in that announcemen has since expired. In addition to the rates listed there, there are a couple of new rates.
One is on
Banda Ancha Telcel - Paquetes Internet Telcel Amigo :
15MB valid for one day for 15 pesos - good if you just want to check you have usable reception (or for minimal email)
and when you've signed up for online access to your account at
https://www.atencionenlinea.telcel.c...ntes/index.jsp
there are three more low-volume deals (for those who just need the bare minimum smartphone email checking etc)
50MB or 30 days for 47 pesos
150MB or 30 days for 143 pesos
300MB or 30 days for 286 pesos
(note: those are border-area prices, it's little more elsewhere)
In some areas (like where I am!), the 3G network is now getting overloaded and unreliable, but otherwise the speeds you are getting are fairly typical, although latency is much lower near the northern border.