Was experimenting with fuel harvesting bases like this yesterday, firstly while the habitat is huge it's very cheap, costing only 200 spacebucks out of a 9000 spacebuck facility. Secondly, it's cheap compared to the cost of fuel refinerys. This is the design I came up with:
FuelHogsmall class Fuel Harvester Base 797,800 tons 2280 Crew 9180.2 BP TCS 15956 TH 2880 EM 0
180 km/s Armour 1-766 Shields 0-0 Sensors 1/1/0/0 Damage Control Rating 1 PPV 0
MSP 7 Max Repair 48 MSP
Intended Deployment Time: 3 months Spare Berths 1
Habitation Capacity 50,000
Fuel Harvester: 200 modules producing 4000000 litres per annum
Faulkner & Lyons 240 EP Commercial Ion Drive (12) Power 240 Fuel Use 4.05% Signature 240 Exp 4%
Fuel Capacity 8,000,000 Litres Range 44.4 billion km (2857 days at full power)
This design is classed as a Commercial Vessel for maintenance purposes
This design is classed as an Orbital Habitat for construction purposes
Some explanation of design choices.
200 modules is enough to double my starting fuel production, 180km/s can get to Jupiter in only about a month which is fine, tanks for 2 years production is ok I guess. the design can be made in about 2 years with starting factories, I would probably use about 20% production though and wait for the long term, I won't run out for fuel until my fleet is grown and starts training anyway.
Most importantly it costs this:
3475.7 Duranium
5 Corbomite
4530 Boronide
593 Mercassium
576 Gallicite
While 200 refineries costs: 24000 duranium. So if you have Boronide to spare and want to save duranium then harvesters are great, I would probably even add a few more layers of armour for only a couple more thousand.