Although not unethical, it may not be the wisest way to proceed either.

The risk is, you buy $1800 tickets, visa comes through in time, and the $600 fare is no longer available. (IOW, $1200 risk if visa comes thorugh; no money risked if visa does not come through.)

I think a better solution, also suggested by others above, is to buy restricted tickets which allow you to change dates for a penalty. That way, if the visa comes through, you already have the tickets you want to fly. If not, you'll pay something like $100-200 to rebook. (IOW, no money risked if visa comes through; $100-200 risk, if the visa does not comes through.)

There is immense expertise here about airlines and airfares. Travel agents and airlines often don't bother to look deep/wide enough. Tell us the cities and dates and you may be surprised by the suggestions you'll get.