If you loved the player, that's the main reason to get it. A jersey isn't a financial investment, it's something you wear because you like it, and if that guy meant something to you while he was on the team, the shirt's going to have sentimental value to you no matter where he is now. Plus, a lot of jerseys from historic or beloved players who left end up being more iconic than the ones from guys currently on the team.
What you should think about is whether you're actually going to wear it. If you'll put it on to go out, to watch matches, anywhere at all, go for it. But if the idea is just to stick it in the closet and leave it there, then maybe it's not worth it. The low price they're offering you is exactly because the market drops when a player leaves, so if you ever wanted to sell, you'd probably get something close to what you're paying now.
My advice: if you've got the money and the jersey appeals to you, buy it. It's way better to have something you like and that brings back good memories of the team than to be wondering "what if I'd bought it". Old jerseys have their own charm, and honestly, serious fans often prefer jerseys from players they loved back then over the ones from current players.