Unless you need or REALLY want a smaller weapon, or absolutely must have a Gen4 for one of its new features, a G19 holds no practical advantage over a G35 with a 9mm conversion barrel. With your G35 combo, you can shoot the cheaper ammo for practice or target shooting, and go back to the more powerful .40 ammo for defense or shooting in competitions that give an advantage to the bigger bullet. So, except for the first few possibilities I listed, I see no objective reason that this trade would be a good idea.
If one or more of the above reasons apply, then there is another thing to take into account; a few of the Gen4 Glocks have had functioning problems, especially with low-powered target ammo. Can you test-fire the G19 before committing to the trade? Are you prepared to feed it more expensive full-power ammo if that's what it takes to get reliable functioning? If not, then I'd pass; personally, if I ever traded-off a reliable dual-caliber gun for an unreliable single-caliber one, I'd be kicking myself around the block every day of the week, and twice on Sundays.
As for the cash amount -- LNIB is still used, and unless your G35 is pretty beat-up, with the extra magazines and the spare barrel, I'd want at least $200, AND a test-fire session with the G19 before the deal was finalized. If your gun is well-used, I might lower the amount of boot required to $150. If anything less than that was proposed, I'd tell him "I guess I'll just save my pennies until I can buy a Gen4 G19 outright." But that's me, and it's your call.
Let us know how it works out!
"Placement is power" -- seen in an article by Stephen A. Camp
(RIP, Mr. Camp; you will be remembered, and missed)