The amortized cost for an apples to apples (couldn't resist) comparison is:

Imac is  $1879 plus taxes per year over 4 years is ~$531/year (see current budget)

IMac 21.5 2560-by-1440
2.9GHz Intel i5
16GB Ram
1TB 7200rpm Drive,

(note) we would have to spend an additional $80 per iMac for a new security bracket
since the new iMac stand is too wide for the old security plate.

Mac Mini pricing from http://store.apple.com/ca_edu_108369/configure/MD388LL/A
$1364 plus taxes over 4 years gives $386/year
Specs:

• 2.6GHz Quad-Core Intel Core i7
• 16GB 1600MHz DDR3 SDRAM - 2x8GB
• 1TB Fusion Drive
• User's Guide (English)
• Accessory Kit
• NO mouse, keyboard or Apple Care as we'd purchase them no matter which Mac we purchase.

Monitor Pricing from http://www.canadacomputers.com/product_info.php?cPath=22_700_1104&item_id=042094 and assumed to last 8 years
$650 over 8 years gives $82/year

Mac Mini + cheapest 2560x1440 monitor is $386 + $82 or ~$468/year.

At 60 per year, we're talking $43X60units or less than $3k/year difference. (about 1 to 2 units of which ever).

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So what did we end up buying
Mac Mini 2.6GHz i7 16GB Ram, 1TB 5400rpm drives +Applecare $1139.00+$99.00=$1238.00
MS keyboard and mouse combo $14.50
Asus 27" monitor (2560 X 1440) $700.00
Datamation's DS-MCMIN-2.0 #0050 locking plate @ $59.00
CPM-211 Steel Cable Kit - Black-KA for monitor lock @$36.00

Total  mac mini (including physical security components etc.) cost is $2047.50