Thank you for your comments!

I have used Windsond up to 2.5km altitude. The current solution can do higher but I haven't checked exactly how high. I understand you want as high as possible but at some point the increased technical demands starts affecting the price of a sounding. If there's much interest in this I'll look more into it and give a better answer.

I'm aiming for a price point under $150 per unit. The exchange rate is unfavorable for USD at the moment.