What do backlight modded gba's go for now in days.
$100 and up depending on the seller. Given the price of the parts, and the rest I broke it down when someone asked me on another board maybe a month ago and $100 is a fair price. Given you have a soldering iron with little experience and can use a dremel or other tool to carve out the inside of the original GBA the effort takes a few minutes at best for the soldering/prep and attaching the ribbon cable adapter and new panel. Most the time goes into whatever tool you have to carve the plastic.
GBA itself is like a $2-20 online/offline find, the LCD is $50~ with the adapter ribbon cable. It's possible the GBA you find is jacked up, a replacement shell with a glass screen is like $15 (or $13 for a OEM like easily scratched plastic one.) Add all that up on the high you're around $80-85 (or -15 without the new body for it.)
I've seen a lot of greedy assclown mass makers on ebay want upwards of $150-200 for them shipped which is a joke as no one needs lawyer like rates for a few minutes of modding work. Just look for a private seller and you'll pay $100 or a few over that if you're patient.