You can build your farm using 5g smartphones or 4g USB-sticks. Phones are easier to buy, set-up and maintain. Also 5g can provide you with 100 -300 mbit speeds on your proxies.
4g-stick farms are slightly cheaper, but you need some time and knowledge to build and maintain such farm. 4g will provide you with 30-100 mbit speeds.
The cost of making own mobile proxies consists of two things:
1) One time REFUNDABLE spending - buying 5G smartphones or 4g sticks and equipment for connecting them
That's 90-120$ for a used 5G phone
20-50$ for a 4g stick and about 1000$ for equipment to make them work together
2) Monthly payments for connections
Payments for simcards are 90% of monthly costs. You can ask for cheaper tarrifs with your career. Usually they have discounts for companies when you buy many simcards. If you use smartphone - you will have to pay for a specific software to convert them to proxies(that'll be about 5$ monthly) If you use usb-sticks to build a farm you will also need a landline with a static IP to connect, that could be expesive!
So if to speak about 50 devices:
For 5G smartphones:
One time costs(50-90% refundable cause you can sell them): 50 5Gphones*about 110$ =
TOTAL ONE-TIME COST about 5500$
Monthly costs(for average european market): 50unlimited plans*20$ = about 1000$ + software payments 50phones*5$=250$ + electricity about 20-30$ monthly
TOTAL recurring cost is: 1250-1300$ monthly
For 4G usb-stick farm:
One time costs(50-70% refundable): 50sticks*about 30$ = 1500$ + server and equipment = 1000$ ==
TOTAL ONE-TIME COST about 2500$
Monthly costs(for average european market): 50unlimited plans*20$ = about 1000$ + 1gbit landline with fixed IP for server=150$ + electricity about 50$-70$ monthly
TOTAL recurring cost is: 1200-1300$ monthly
These prices may significantly vary depending on your country.