The smaller number can cost more
Seeing 20 beside 90 can make 20 look cheaper. That conclusion assumes both figures measure the same purchase. They may not. A currency code identifies the money; a unit identifies what that money buys. Until both are explicit, ordering the displayed numbers answers a different question from ordering the prices. The examples below are illustrative calculations, not reported test results.
Convert the whole price
Suppose one supplier quotes EUR 90 for a case of six identical items. Another quotes USD 20 per item. Assume EUR 0.90 per USD, no conversion fees, and tax and delivery included in both prices. The first offer costs EUR 15 per item; the second costs EUR 18 per item. The apparently smaller figure is more expensive. Currency conversion handles the money; dividing the case price by six supplies the common unit. For six items, that difference becomes EUR 18, even though the raw figures suggested the opposite ordering.
Check what the amount covers
Before deciding, write each offer as a sentence: this amount, in this currency, pays for this quantity, over this period, including these charges. Normalize both offers to the same basis. Record the exchange-rate date and verify each quoted package size. Multiply by the original quantity and reverse any currency conversion to recover the quoted amount, allowing for rounding. Confirm whether a displayed payment is a full price, a deposit or an installment. Arithmetic can be exact while the comparison remains incomplete.
Hypothetical boundary case: a deposit
Imagine two offers for three identical items, with no additional charges. Offer A shows a EUR 45 down payment and requires another EUR 120 later. Offer B costs EUR 150 in full. A totals EUR 55 per item; B totals EUR 50. Matching currency and units still leaves payment scope unresolved. Decision rule: rank total purchase cost only after including every required payment; if later obligations are unknown, withhold that ranking. For immediate cash needs, compare amounts due now separately.