Need help with extremely complicated (for me) MySQL query

I have a very difficult (for me) mysql query I'm trying to write -- I'm so very very close besides one little part. Here's the deal-- I have a table of transactions and a table of plants, and I'm trying to get a "leaderboard" together which will tabulate all of the transactions and the costs and calculate total revenues and开发者_如何学Python profit margins. Here's the relevant fields (modified names to make it simpler here):

TABLE transactions:

username | user_city | total_sale | qty_sold 
============================================

TABLE plants:

plant_city | plant_base_cost

So I've been able to calculate everything I need to know about the sale with a pretty good-sized query, which results in this (two example rows):

username  |price |  user_city       |  plant_city  |  base_cost | qty |  cost |  profit
=======================================================================================
AndyHabs  | 1140 |   1           |  1           |    65   | 6   |  390  |   750
AndyHabs  | 1000 |   1           |  5           |    100   | 5 |  500  |   500

Pretty simple, right? I just need to sum the relevant fields and group them by username--except here's the issue! If the user_city is DIFFERENT than the plant_city, I need to multiply the base cost by 1.6... I have NO IDEA how to incorporate this condition into my query--it was pretty simple to perform a basic calculation on a field, but is there a way to incorporate a condition like this in my statement?

I have no problem inherently just doing this in a PHP script--but if this table gets much bigger it'll take a really long time, and I would love to use some nifty pagination scripts which would make it nice to just get the data I need straight up from MySQL.

Thanks in advance!

---

Use a CASE expression in the calculation.