You are way off base on the cost difference between the two systems all things being equal assuming you are buying a complete new system. First the difference in the cost of the two systems is the difference in the cost of the camera body and the housing. The ports, gears and lenses you chose will be the same with both systems, so no cost difference.
At B&H the cost of the E-M5 body is $999.00 and the E-M1 body is $1399.00 so a $400.00 difference. The Nauticam NA-EM5 housing is $1450.00 and the NA-EM1 housing is $1850.00 so another $400.00 difference for a total of $800.00 system to system all else (lenses, gears and ports) being equal.
Now you need to compare what you get with each housing. The E-M1 housings comes with the grips, 1" ball heads and stainless handle brackets as well as the vacuum system electronics built in. The E-M1 housing also has vertical shutter trigger and AEL/AFL trigger for rear auto focus along with several more function buttons for the E-M1 which is the most programable camera on the planet.
At the very least the E-M5 housing would need the Flex II tray, second grip and "1 ball heads to mount dual strobe arms. Flex tray $130.00, second grip $40.00, two 1" balls $16.00X2=$32.00 for a total of $202.00. Add the handle brackets $60.00 and the vacuum electronics around $220.00 installed by local dealer and now you are at $482.00 in add-ons which are standard with the E-M1 housing. Both housings would require the vacuum valve and pump about $220.00 for the complete vacuum system. I also have the $30.00 wrist strap with my E-M5 system because I hold the housing with my right hand to allow better assess to the shutter release rather than holding the right grip. Just the ability to use a vertical trigger and hold the housing by the grip makes the E-M1 housing very compelling.
So with tray only you have about a $602.00 difference between systems. Total difference with all items added to both both housings would be around $318.00. If you add the wrist strap you are now under a $300.00 difference system to system.
I own both systems and I can tell you that the E-M1 system is head and shoulders above the E-M5 system in terms of ease of use, function access, AF triggering and more. The difference in size between the two systems is not that great.
I realize you can probably find bigger discounts on the E-M5 than you can on the E-M1 but then the E-M5 is nearing the end of its run and the E-M1 is just at its beginning.
You can also get over $300.00 in incentives with the E-M1, with the 12-40 zoom I got a $200.00 rebate and the MMF-3 ($180.00) adapter for 43 lenses which work very well on the E-M1 and not well at all on the E-M5.
The E-M1 has been chosen camera of the year by several highly respected organization and is a true "pro" camera by definition. I think the E-M5 system is a great value with excellent image quality but the E-M1 system is truly a step above and it could be argued that it is a value compared to DSLR systems with the same image quality and less features.