Well for almost all of it, I used maple/papaya seeds which average around 9.5 gp/xp including paying farmer, but I assume you can afford magic/palm :x They're more like 15 gp/xp paying farmer. E: May not be accurate with xp weekend coming soon.
There's also watermelons, for around 10k xp per 80 minutes. Based on 1766 patches of mine, they're around 5 gp/xp. You should plant either nasturtium or white lily in the flower plot for these, you only need to grow it once and not pick, and its protection lasts forever.
Dwarf weed or fellstalk would be main two herb seeds, depending on what you want. And you could harvest whiteberries, cactus spines, morchella to save money and get a little extra xp, although tbh the time spent harvesting them can be better spent on other gp methods.
And if you can semi-afk for a bit, Livid is 33048 farm xp/hr assuming absolutely no server lag. I forget how many runes you use for that though, but probably still relatively cheap.. :3 And you can use wishes from it for an extra 1-use allotment patch and/or protect a patch scroll. I used the scrolls on magics during my final month; you take 250 noted coconuts, use the scroll on the patch, it takes 25 coconuts, and protects the patch from disease (no need for further payments) for that magic tree and the following 9 magic trees. I think if you plant something different then the protection expires.