How to make GPS/GPRS tracker and real-time updater cheaper?

i'm getting started arduino , want make portable, slim, gps tracker gives real-time updates , activates either when object leaves "safe zone" or when it's texted phone.

a stretch goal device emit high pitched tone, or other alarm sound notifying people passing by.

it seems need following parts:

• tinyduino $20 - eventually, "lilyboarded" atmega328 chip $3.
• gps/gprs module $60 - obtains positional data/connects internet relay information in real-time.
• antennae x2 - 2 needed gps , gprs?
• sim900a or sim809 - allows texting , 2g or 3g data using pre-paid sim cards. essentail real-time updates , text abilities.
• power source - 3.7v lithium ion battery
• container - tictac now.
• misc. case parts
• speaker module - optional

i've found few tutorials, including 1 want start with,
and 1 has real time gps/gprs, parts on on $100...


questions:
1. how make cheaper?

how can device like 1 be $13 , same things? there cheaper way or because sourcing parts directly cheap factories in china? it's tempting buy 1 , reverse engineer pieces...

2. need 2 antennaes?

3. there way use less parts cost down? i'm asking because there may devices accomplish similar things i'm not aware of newbie

sim908. gps/gsm/gprs in one. still need 2 antenna (one gps, 1 gsm/gprs), it's large (forget tictac!) , power hungry. applies gsm/gprs boards.

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