"high-end clientele" is to "low-end clientele" as "carriage trade" is to what?

I came across the phrase bargain-basement while researching for an appropriate answer.

(adj.) markedly inexpensive (bargain–basement rates)

[Merriam Webster]

From The 8 Steps: Your Business or Your Life - Getting All You Want Out of BOTH:: By David Shepherd, customers can be broadly classified into 4 groups (The Harvard Model):

Thus, I feel bargain-basement customers would better suit your context.

I would suggest mass market. According to Wikipedia:

The mass market is the largest group of end consumers for a specified product. It is the opposite of the term niche market.

The carriage trade is a niche market, but obviously there are other niche markets, so if you want a term to use in isolation (i.e. not mentioning carriage trade) then it wouldn't fit well. But it would work well in e.g.:

Most shops maximise their profits by aiming at the mass market: "pile 'em high and sell 'em cheap". Some, however, manage to make more money by aiming at the carriage trade, by selling fewer items but selling each one at a significantly higher profit.

The term budget is used to refer to customers who generally have a limited and predetermied amount of money to spend. Without referring to low-social classes, I'd say that "budget customers" are those that may convey the idea of low, careful and restricted spending, just opposite to what the big spenders, like the "carriage trade", convey.

Budget:

• the amount of money that is available for, required for, or assigned to a particular purpose.

(M-W)

It's not an exact match, but you could consider using "Trade (business) with bargain hunters." A bargain hunter means:

a ​person who looks for a ​place to ​buy something at a ​price that is ​cheaper than usual.

[Cambridge Dictionaries Online]