I browsed through my collection of COMPUTE! and COMPUTE!'s Gazette and found advertisments from Mimic Systems in the following issues:
December 1984: no RRP given
January 1985
Spartan base system (including BUSS, CPU and DOS cards) $599
Additional BUSS card $299
Additional CPU or DOS card $199
Price comparison: Protecto Enterprizes sold a bare C64 for $175. Harmony Video & Computers in New York sold an Apple IIe with disk drive for $844. They were the only advertiser to publish a price.
February 1985
Spartan price list same as in January. Protecto's C64 still at $175, but Harmony's Apple IIe had been raised (!) to $859. Still no other retailer willing to let their competitors know how much they charged for Apple systems.
March 1985
Spartan price list still unchanged. Protecto still pushing C64 at $175. Harmony's Apple IIe still at $859. Rest of the retailers only offered price quotes after a phone call.
April 1985
Spartan price list still unchanged. Protecto dumped their C64 to $139! Harmony also had a price drop, Apple IIe at $809.
June 1985
Last issue that I've got that has a Spartan advertisment. Their price list still the same as in January. Two page advertisment for the Apple II series, but no RRP in that one. Anyway, Protecto's C64 still at $139 while Harmony's IIe with disk drive again had been raised to $819.95!
Now, I know the Spartan just adds II+ compatibility, not IIe and there were quite a number of advertisers who might have offered better prices upon a phone call, but roughly speaking it seems a C64 + Spartan initally would cost around $775 plus shipping, disk drives, monitor etc, dropping to $740 in the late spring of 1985. A true Apple IIe including a disk drive could be had for at least $860, later dropped to $810 at best. So yes, if you add in the disk drives and other equipment required to actually have use of the Spartan with a C64, the total cost would be quite a bit higher than what you could buy a real Apple for. However if you already got a C64 and just wanted to expand with an Apple II system, I suppose you could save at least $50-$100 by getting the expansion unit instead of getting a second computer.
Update: So the eBay auction just ended at $1552. Even after adjusting the $599 RRP for 27 years of inflation and taking into consideration the lot on eBay included a floppy drive you have to buy extra, I'd think it just sold for more than it once cost...