new weapons and extra unpgrades were overpriced at the beginning.
even if you save up some money at the earlier levels, you barely have enough money to purchase one of the basic upgrades/weapons. your spaceship soon gets overwhelmed by the sheer amount of missiles/lasers aiming towards you after level 4. (those yellow spaceships shooting in all directions are just too much to handle, especially when you only have one or two upgrades). this can be very discouraging for new players, as the firepower between you and your opponents are not well balanced at that point.
although you can actually earn some points even if you get defeated in a battle, it can be very tedious to slowly build up the power of your spaceship as you have to basically replay the same level over. instead, you should either reduce the cost of upgrades/equipments, or insert more levels while allowing us to select current levels and include bounses to make items more accessible to you.
to maximize the chances of winning, you have to use different stragegies based on types of enemies your engaging (especially for bosses). it would be helpful to players if you describe the characertics of enemies you would see and tips on surviving waves of assault should the player lose the level.
and as with other shooter and similar games, there's an element of luck to the game. firstly, it is difficult to predict the effect/power each piece of equipment /power of upgrades if you haven't purchase one yet. sure, you might spend precious bytes on upgrades/items that offer relatively little advantage on enemies over other upgrades. i think it is worth considering to include a 'trial' option where you can experiment with different upgrades and equipments to help you decide which aspect you need to spend points on. secondly, the chance of winning a level depends quite heavily on the number of times you receive powerups on the way. the position where those powerups appear should be fixed so that player of roughly the same chance of winning each time he replays a level.