How hard is it mod the antennas and how much more distance would you realistically get? I can't afford the advanced model really
It would be a little complicated and involve disassembly of the raidio control and modding it so you can use an external antenna assembly.
Depending on how much you are willing to spend on good antennas, on the low end you can add around 25% to your range and on the high end going all out you can get as much range as battery life. At that point tho it would have been more cost effective getting a P3 advanced
So cost break down, low end cost.. Roughly $20 for pair of wifi antennas and ufl or mcx to rpsma pigtail cables. Mid range roughly $100 for something like a DBS mods antenna, one which works for the Phantom Vision line will work for it. And on the higher end for max range you'll want to add power amplification so now you're looking at an over $200 spend.
What you loose by not just getting a P3 advanced is no Glonass, (Russian gps) motion flow, shutter and video controls right on the remote without having to use your tablet. Lightbridge for the control and fpv offers a more reliable connection than P3 Standards wifi based connection. Lightbridge also will reconnect more quickly if and when you do fly outside control range. Camera optical quality may not be as good as the advanced and does not do 1080p 60 FPS. Not to mention once you begin doing mods you loose some of the compactness of the controller setup for the external antenna mods. There's also the chance you may break your remote if not careful while doing the mod.
Again if range is a concern, don't go with the standard, save up a little longer until you can at least go with the P3 Standard, and you'll get those additional benefits of the Standard as well.