Here are some comparables from Namebio. As you can see "xxxpay.net", where xxx is any word, domains don't sell very frequently, only 8 in the last 4 years. IF you find a buyer, expect about $1000-$2000. This is only going to be bought by an end user looking to set up a payment company. Dotcom version of same name would be worth about $3000-$10000.
Good luck
Domain Price Date Venue
epay.net 2,306 USD 2016-02-28
GoDaddy
cpay.net 150 USD 2016-02-12 Bido
upay.net 1,515 USD 2015-07-03
GoDaddy
expresspay.net 1,495 USD 2014-11-26 Sedo
bitpay.net 5,304 USD 2014-02-19 Sedo
bestpay.net 2,188 USD 2012-12-30 Afternic
lawpay.net 1,277 USD 2012-11-07 Afternic
youpay.net 1,461 USD 2012-03-14 Afternic
cloudpay.net 2,500 USD 2011-07-27 Sedo
textpay.net 2,088 USD 2011-04-20 Afternic
Domain Price Date Venue
neatpay.com 2,500 USD 2016-11-18 Sedo
channelpay.com 4,500 USD 2016-10-09 Nidoma
billapay.com 2,500 USD 2016-09-30 Sedo
egopay.com 2,026 USD 2016-09-25 Sedo
secupay.com 16,726 USD 2016-09-21 Sedo
remitpay.com 3,200 USD 2016-08-31 BuyDomains
heropay.com 4,039 USD 2016-08-08 DropCatch
skinpay.com 3,000 USD 2016-04-11 Uniregistry
bepay.com 5,100 USD 2016-03-02 Sedo
vibepay.com 2,000 USD 2016-02-06 BuyDomains