Here's what wikipedia has to say about 'economy rice':

<em>Economy rice</em><em>, found in </em><a href="https://en.wikipedia.org/wiki/Singapore" rel="noopener noreferrer" target="_blank"><em>Singapore</em></a><em>, </em><a href="https://en.wikipedia.org/wiki/Malaysia" rel="noopener noreferrer" target="_blank"><em>Malaysia</em></a><em> and </em><a href="https://en.wikipedia.org/wiki/Thailand" rel="noopener noreferrer" target="_blank"><em>Thailand</em></a><em>, refers not to a specific dish in the Singaporean, Malaysian and Thai pantheon of cuisine, but rather to a type of food or a food stall commonly found in </em><a href="https://en.wikipedia.org/wiki/Hawker_centre" rel="noopener noreferrer" target="_blank"><em>hawker centres</em></a><em>, </em><a href="https://en.wikipedia.org/wiki/Street_vendors" rel="noopener noreferrer" target="_blank"><em>street vendors</em></a><em> or </em><a href="https://en.wikipedia.org/wiki/Food_courts" rel="noopener noreferrer" target="_blank"><em>food courts</em></a><em> in these countries.</em>

<em>Economy rice stalls typically consist of a case containing anywhere from 10-15 troughs of cooked food, including meat, vegetables, eggs and </em><a href="https://en.wikipedia.org/wiki/Tofu" rel="noopener noreferrer" target="_blank"><em>tofu</em></a><em> dishes. Customers select any combination of these dishes, which are served accompanied by a portion of steamed white rice. In Singapore, it is more common to find the food on open troughs kept warm by hot water and an electric heater below.</em>

It may not be relevant here, but I agree with Mr T. W. Guy that 'economy rice' can be a branding.

Supermarkets these days have at least three different ranges of the same basic product: as well as their standard range, most retailers have a premium range at a higher price and an economy range at a lower price. So if you want to save money, you don't buy luxury top-of-the-range rice (very expensive) or even mid-range (medium price) rice, you buy economy rice.

I have no idea what 'economic rice' might mean. It doesn't make much sense to me.

Couldn't "economy rice" be used as a branding? After all, we have "economy class".

My mother (sell) SELLS (economic) CHEAP OR INEXPENSIVE rice every day. 

The adjective "cheap" means low cost but sometimes it also means "poor quality" like if we buy cheap clothes or shoes and they do not last very long because they fall apart.

The adjective "inexpensive" just means "not expensive" and it does not refer to the quality (like the word cheap which means low quality)

Economic. Or cheap. Economy makes no sense in any case.

Wendy you need to provide the phrase in a sentence.

In other words, tell us how you want to use the phrase, then we can tell you which one is appropriate.