I would figure the cost of whatever kind of building you would replace that one with if you had a loss. You may not have a need for as large a replacement structure? maybe a 30 x 50 pole type bldg. would cover yor needs/ Your agent should better serve ya by doing some ground work, thay are typically undertrained and or looking for an easy way out.

I used to sell property/casualty insurance specializing in farms but times have changed in the laST 20 YEARS! I will say few ag buildings are insured to their repalcemenmt value due to the premiums..

Considering how much the price of steel has gone up over the past few years, and considering that it is VERY unlikely to go down, the question is whether you want to replace the Quonset with the same thing or with an equivalent, and if you want to insure it for the present value or for a full replacement value. If the building is central, vital or necessary to your operation, then I would consider getting full replacement insurance - although probably I would, if it became necessary, put in a pole or stick-built barn rather than a Quonset - I think that would prove the less expensive at today's (or tomorrow's) prices. If it is a building you use daily, or even weekly or seasonally, then I would go for the higher value. Costing out comparable buildings in the area might help make up your mind. If course, that is what the agent is supposed to do, but......

I do think your lower value is not realistic in today's market, but her higher one may be as unreasonable - building values/costs are so subject to regional variations that I can't say strongly for one or the other.

This web site lets you price different types of steel buildings, including quonset huts:
http://www.buyerzone.com/industrial/steel_buildings/qz_questions_790z.jhtml?kw=Quonset%20Hut&_requestid=951737