My question is why is the Ultegra di2 so much more expensive than the Ultegra mechanical. $6200 vs $3500? That is crazy! No way the wheel/cockpit/saddle spec could be bumped enough to make up the difference. Is it a totally different layup as well? (other than the obvious electronic vs. mechanical frame change)
The AR4 and AR2 frame, fork, and seatpost are indeed different in terms of lay up and frame molds.
I know you don't have the frame, fork, seatpost, and derailleur hanger cost differences. Let's ignore them for the purpose of your question.
Please help me calculate the price differences of:
HANDLEBAR 3T TORNOVA TEAM STEALTH vs. FELT Flat top Aero Road handlebar ($70)
STEM 3T ARX II TEAM STEALTH vs. FELT SL 3D forged 6061 Aluminum stem ($45)
TOP CAP Carbon w/ 7075 bolt ($30) vs. Aluminum w/ 2024 bolt ($15)
SHIFTERS ST-6870L vs. ST6800L
ST-6870R vs. ST6800R
F/DER FD-6870 vs. FD6800F
R/DER RD-6870-SS vs. RD6800SS
SADDLE Prologo Zero TiRox vs. Prologo Zero T2.0 Cr-Mo
TIRES Diamante Pro Light vs. Rubino Pro Slick
WHEELS 3T ACCELERO 40 CARBON TEAM STEALTH vs. Shimano WH-RS31
The AR2 also includes:
VALVE CAPS: 7075 CNC Machined 30mm Valve stem extenders
BATTERY CHARGER: SMBCR2
DERAILLEUR CABLE SET SM-EW90-A, Di2 for Standard Handlebar specification (E-Tube Port x 3), 6 x EW-SD50-I for built in type wire routing, SM-JC41 junction for internal battery & cable routing
I'm crystal clear on where the OEM price differences are of these components and why there is a $2500 price jump from the AR2 to the AR4 but perhaps there are aftermarket sources that dip well below the MSRP.
I can add on the cost differences on the frameset and the penalty that the small volume of Di2 production adds vs. the huge volume of mechanical Ultegra bikes but I think you'll find these differences are absorbed and the component price differences are higher than you anticipated.