Each upgrade seems to give 10% increase in base hp and attack, but there is some exceptions.
1. The rocket tower have 250 base damage, but for every attack upgrade it receives 20+ damage instead of 25+.
2. The HQ laser does 79 base damage and get 6+ for each upgrade instead of 8+
3. The marines does 35 base damage, and get 3+ for each upgrade
4. The ghosts does 150 base damage, and get 20+ for each upgrade instead of 15+
I hope this information was useful.
(I set 8.74 prefix, but appeared as 8.73c)
1. The rocket tower have 250 base damage, but for every attack upgrade it receives 20+ damage instead of 25+.
2. The HQ laser does 79 base damage and get 6+ for each upgrade instead of 8+
3. The marines does 35 base damage, and get 3+ for each upgrade
4. The ghosts does 150 base damage, and get 20+ for each upgrade instead of 15+
I hope this information was useful.
(I set 8.74 prefix, but appeared as 8.73c)