Finding reduced costs in AMPL

Question

I'm trying to grok linear programming with AMPL and I'm having trouble finding some information. I found the cutoff value of export_price where it starts to pay off to export products, by prod_trans.rc. The inital value of export_price was zero.

Now I want to find the cut-off for import_price, with the inital value is zero as well. I could just set the initial value to some large number M and look at the reduced costs again. That way, the cut-off will be M minus the reduced cost. But this seems inelegant and intuitively, I feel there must be a way to read off the cut-off value even when the import price is set to 0. Is there a way?

This is the relevant part of the objective function:

// irrelevant terms
+ sum{ m in IMILL , p in PRODUCT ,c in EXPORTS}
     prod_trans[m,c,p]*(-export_price) +
  sum{ c in CITY, p in PRODUCT, m in IMPORTS}
     prod_trans[m,c,p]*import_price;