I advise you to watch eBay. In the past few weeks, off of eBay, I've gotten the Tarakava for $20 shipped and the Muaka and Kane-Ra for only $19.74 shipped. I think the best option is not to burn the money immediately on one large purchase, but to spend a little while prowling the eBays (like me) until you find the best deals, to get the biggest bang for your buck. I'd also recommend not buying new-in-box items, as they are far more expensive, while guaranteeing little more.
If you're asking advice on what items to buy, this is a largely personal preference. However, I would suggest using that money (if you are sure you want to spend a vast majority of it on Bionicle) to buy the rarer items that are not often seen. For example, the Battle Vehicles, Karzahni, and above all, Toa Mata Nui.
That said, it is more important (in my humble opinion) to spend the extra time to get the best price for the item (especially as some of those rare items can get quite pricey) as opposed to buying it straight away.
There are a few sellers on eBay who routinely sell high quality Bionicles at good prices, but I don't want to give them away
Best of luck!
Steam Name: Toa Onua - always ready for TF2 or a good chat.Xbox Live Gamertag: PluggedSky51173 - Halo 4 for (4) the win!Minecraft Username: ThePoohster - sometimes play on T9K.Wants: 2003 Jaller (from Jaller and Gukko set), Exo-Toa, Turaga Nuju, Turaga Vakama, Black, White, and Grey Krana, Heat Vision, Sonic, Fire Resistance, Power Scream, and Shadow Kraata, Axonn, ORANGE FIKOU.
I got rid of my picture, are you happy?