Suggestions about sub/amp in ford mustang 2017

So currently it has stock six speaker setup with stock head unit. My nearby electric store suggested me hertz 1000watt sub worth $269 and a pioneer 500watt sub worth $140 brand new. The sub is in the box as well and wiring for $60 and labour $147 abd speaker mounts for $60. He said since you are getting full system. He will hook me up with some discount worth $60-$70 and it will cost whole between $700-$900. So my question is is it good deal and if anybody has this system and it it good enough and it it worth getting.

Get multiple quotes before having anyone do any kind of work.

Keep shopping. All you need is one sub in a good cabinet and an amp. Look only at the power rated as RMS watts. If it's a hardtop car, 500 rms watts is good unless you want all the neighbors to hate you for shaking their windows a block away.