@ Lion66six: It may be a good Idea to turn on "Show Funds" in options geoscape section, to always see how much money you have.
How Reactions work:
Alien and X-Com Units will undergo a reaction check if they walk into each others cone of vision, there is one excepting the "Mutual Surprise Rule", which makes sidestepping extremely increasing the survivability of soldiers, with "Advanced Movement Methods".
The reaction check compares the following values:
Number of Reactionsstat value + Time Units current value.
E.g.: A alien walks into the visual field of 3 xcom soldiers. Soldier 1 has 80 TU and 70 reactions, Soldier 2 has 70 TU and 50 Reactions and Soldier 3 has only 20 TU (from 80) left but has 100 reactions.
In our case we just assume that all 3 soldiers see the alien at the same time, alien is a Muton with 60 TU (from 64) and reactions 70.
Reactioncheck values:
Muton: 130 (60 TU + 70 Reactions)
Soldier 1: 150 (80 TU + 70 Reactions)
Soldier 2: 120 (70 TU + 50 Reactions)
Soldier 3: 120 (20 TU + 100 Reactions)
Muton moves into sight range of soldiers, having 130 Reactioncheck value, Mutual surprise is not applied, because the soldiers are not directly facing him horizontically.
Soldier 1 will Fire a Snapshot with his Laser Rifle at the Mutons, since his Reactioncheck value is higher then the value of the Muton. The soldier spends 20 TU and reduces his Reactioncheckvalue to 130.
What will happen next?
Soldier hits the Muton, gaining his attention or the soldier misses the Muton and it will walk further. For the next reactioncheck it does not matter.
Soldier 1 and Muton have currently both a Reactioncheckvalue of 130.
As soon as the Muton does any action, Soldier 1 will fire at the Muton again (as long as he is not killed by the Muton with his next action).
We now assume that Soldier 1 is a terrible shot and misses the reactionshot. The Muton then walks further and Soldier 1 can fire again a Snapshot.
Reactioncheckvalues: Muton 126, Soldier 1: 110.
Soldier 1 misses again and the Muton continues his patrolling. The Muton spends 8 more TU, reducing his Reactioncheckvalue from 126 to 118.
What will happen now?
Soldier 2 and Soldier 3 have so far failed all the reactionchecks, because they only have a Reactioncheckvalue of 120, but now it is their time!
Due to the fact that Soldier 3 has 100 Reactions, he will fire first, Reducing his TU to 0 and his Reactioncheckvalue to 100, Soldier 3 now can no longer react -> TU: 0.
Soldier 2 with TU 70 and Reaction 50 will fire after him, spending 20 TU to fire his Laser Rifle, reducing his Reactioncheckvalue to 100 -> (TU: 50 + Reactions 50).
Soldier 2 and 3 are both bad shots and miss their Reactionshots, our Muton walks further for 2 fields spending another 8 TU reducing its Reactioncheckvalue from 118 to 110.
Soldier 1 is ready to make a Reactioncheck, he still has 40 TU left and Reaction of 70, so he could still fire twice with his Laser Rifle.
Muton stops and spots Soldier 1 both have still 110 Reactioncheckvalue, since it is the Mutons turns and he passed the Reactioncheck (same value will not fire), the Muton is now Free to fire his Heavy Plasma with a Snapshot hitting and killing Soldier 1. The Muton spends 22 TU firing the Heavy Plasma, reducing his Reactioncheckvalue from 110 to 88.
Soldier 3 can still react and fire his gun, TU: 50 and Reactions: 50. Soldier 3 passes Reactioncheck and fires Laser Rifle Snapshot at the Muton, reducing his Reactioncheckvalue to 80.
Mister Muton gets hit badly and has only 18 TU left, can not fire his weapon and decides to run away. He moves 3 more fields spending 12 TU's and gets shot in the back by Soldier 3, since the Mutons reactioncheckvalue was reduced to 76 and Soldier 3 still had 80.
Soldier 3 is left with 1 kill, 10 TU's and 50 Reactions.
Muton 2 steps on the fields and Snapshots both Soldier 2 and Soldier 3 with a Heavy Plasma killing them and runs away happily grinning
I hope this explains how Reaction works and why TU and Reactions are both important.