You can debug this as follows:
1. Set a fixed value in the following line and see whether you get the value in PostBack:
document.getElementById('<%=txtTotalSplit.ClientID %>').value = '100';
2. If you get the value, that means, you have problem in the following code, and, you need to fix it.
parseFloat(total.toString()).toString();
3. If you don't get the value, make sure the following code returns finds a textbox correctly:
document.getElementById('<%=txtTotalSplit.ClientID %>')
And also, make sure the following method is executed correctly without any error :
totalSplit()
Hopefully, your problem will be solved.