Below are the drawings and pictures of the actuator components.
Most items were custom made in Taiwan.
Prices are what I paid for them there. Shipping was via sea-freight, total cost around  €120.-

Below some pictures of the Taiwan shop: It is a privately owned workshop, with relatively old machinery.
The guy runs the shop with his wife, and he is really a great craftsman. All parts are made very precise, and he also offers his expert opinion on how to build certain parts. Unfortunately he only speaks Chinese, so he cannot provide parts for non-Chinese speaking customers.
 

HTD 8M 22Teeth timing belt pulley without key. Glued onto the shaft where the position sensors are located. 2 pieces needed per actuator. Cost per piece: €14.50

HTD 8M 22Teeth timing belt pulley with 5mm key. Glued onto the main drive shaft.
2 pieces needed per actuator. Cost per piece: €15.80

HTD 5M 72 Teeth timing belt pulley with 5mm key. Glued onto the main drive shaft.
1 piece needed per actuator. Cost per piece: €34.20

HTD 5M 20 Teeth timing belt pulley with 3mm key. Glued onto the DC motor drive shaft.
1 piece needed per actuator. Cost per piece: €10.50

MOD1 54 teeth spur gear. Mounted on the sensor shaft and fastened with M3 bolts.
1 piece needed per actuator. Cost per piece: €10.50

MOD1 24 teeth spur gear. Mounted on the position pick-up pot-meter shaft and fastened with M3 bolts.
1 piece needed per actuator. Cost per piece: €6.50

DC Motor, AMETEK E56617, 50V/1300rpm, 0.375Nm/A, 1 per actuator
Unit price: €34.- (2nd hand)

Actuator shaft. At the end, there is an M10 tapped hole, to which the 10mm rod-end bearings are attached. The bearings used for this shaft are Teflon coated slide bearings.
1 piece needed per actuator. Cost per piece: €5.20

 
Main drive shaft with 5mm key slot. holds the main transmission pulleys.
1 piece needed per actuator. Cost per piece: €3.00

Sensor shaft. holds the front pulleys and position sensor gear. 
1 piece needed per actuator. Cost per piece: €2.00

35mm bearing stud with 17mm inner diameter. Holds the main drive shaft. Fastened with M3 bolts.
4 pieces needed per actuator. Cost per piece: €5.00

17mm ID, 35mm OD bearing for front bungee shaft.
2 per actuator, Unit price: €0.50

Large HTD8M belt, 20mm wide and 1360mm long (170 Teeth)
2 pieces needed per actuator. Cost per piece: €10.00

Small HTD5M belt, 20mm wide and 425mm long (85 Teeth)
1 piece needed per actuator. Cost per piece: €3.90

36mm diameter U-joint, with 18mm inner diameter. Will provide the flexible bottom connection.
1 piece needed per actuator. Cost per piece: €20.70

10mm Rod-end connected to actuator shaft, 1 per actuator   Unit price: €2.20

Currently, total material cost at €1428, so 6DoF is definitely going to be more expensive than 3DoF.