Thanks for your post and he is my opinion.
Actually, it is not just $10.00 per month. You are paying around $29.95 (T-Mobile) to $39.99 (Verizon) for Data Plan in order to keep your device (Blackjack) connected to the Internet.
Cost for Off-Board with TeleNAV: Let's take an average cost of data plan at $35.00 + $10.00 for TeleNAV subscription = $45.00 per month.
Cost of on-board with 2GB memory card with Garmin Mobile 10 as an example: $185.00, 1 time cost
You will ended up paying $540.00 ($45 x 12 months) on TeleNAV vs. $185.00 with On-board solution. You expenses with Off-board solution is going to be very costly if you do it for 2-3 yrs in a row plus you will not get guidance or auto-reroute if you are in an area without cellphone coverage or areas with cellphone coverage but without data coverage.
Map updates is nice but it comes at a cost, which you could spend on another set or equip your spouse or girl/boy friend with another navigation unit with change left. Most maps may be outdated but as long as you know the closest close streets to your destination, you will be okay.
NOTE: Cost of memory card is around $12 for 1GB and $19.00 for 2GB not $50.0o as you have quoted above.