Ask what they are comparing
Prices are rarely equal like for like. Ask which plan and which features they compared, so you answer the real question.
Explain the value in their terms
Name the two or three things this customer uses that the cheaper option lacks. Specific beats a list of benefits.
Decide your price-matching rule
Write down whether you match, when and for how long. A rule agents can apply avoids ad hoc discounts that spread.
Offer other ways to lower the cost
A cheaper plan, an annual discount or fewer seats can bring the bill down without cutting the price. Offer the option that fits.
Let them go gracefully
If the other product suits them better, say so kindly and make leaving easy. They may return, and they may recommend you.
Record what you hear
Tag price comparisons with the competitor named. A pattern is useful to product and sales, not just to support.